Flutter Gradle Plugin: add versionName and versionCode to FlutterExtension (#146044)

This PR is a follow-up of a previous PR of mine:
https://github.com/flutter/flutter/pull/141417. It was unfinished, i.e.
I only implemented it and used it in `examples/hello_world`. No "flutter
create" templates were modified.

This change is theoretically breaking, but in practice, I am pretty sure
nobody uses this. It was not accounced anywhere, the only app using it
is `examples/hello_world`. I did not do that (that=update docs and
templates) because I wanted a solution that is idiomatic in both Gradle
and Kotlin, and only now I found time to do this.

### Without this change

```groovy
defaultConfig {
    applicationId = "io.flutter.examples.hello_world"
    minSdk = flutter.minSdkVersion
    targetSdk = flutter.targetSdkVersion
    versionCode = flutter.versionCode()
    versionName = flutter.versionName()
}
```

### With this change

```groovy
defaultConfig {
    applicationId = "io.flutter.examples.hello_world"
    minSdk = flutter.minSdkVersion
    targetSdk = flutter.targetSdkVersion
    versionCode = flutter.versionCode
    versionName = flutter.versionName
}
```

Idiomatic getter - yay! It's consistent between assignment of all four
props.

### Issue

fixes https://github.com/flutter/flutter/issues/146067

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

---------

Co-authored-by: Reid Baker <reidbaker@google.com>

examples/flutter_view/android/app/build.gradle

@@ -7,24 +7,6 @@ plugins {
     id "dev.flutter.flutter-gradle-plugin"
 }
 
-def localProperties = new Properties()
-def localPropertiesFile = rootProject.file('local.properties')
-if (localPropertiesFile.exists()) {
-    localPropertiesFile.withReader('UTF-8') { reader ->
-        localProperties.load(reader)
-    }
-}
-
-def flutterVersionCode = localProperties.getProperty('flutter.versionCode')
-if (flutterVersionCode == null) {
-    flutterVersionCode = '1'
-}
-
-def flutterVersionName = localProperties.getProperty('flutter.versionName')
-if (flutterVersionName == null) {
-    flutterVersionName = '1.0'
-}
-
 android {
     namespace "com.example.view"
     compileSdk flutter.compileSdkVersion
@@ -38,8 +20,8 @@ android {
         applicationId "io.flutter.examples.flutter_view"
         minSdkVersion flutter.minSdkVersion
         targetSdkVersion flutter.targetSdkVersion
-        versionCode flutterVersionCode.toInteger()
+        versionCode flutter.versionCode
-        versionName flutterVersionName
+        versionName flutter.versionName
     }
 
     buildTypes {

examples/hello_world/android/app/build.gradle.kts

@@ -20,8 +20,8 @@ android {
         applicationId = "io.flutter.examples.hello_world"
         minSdk = flutter.minSdkVersion
         targetSdk = flutter.targetSdkVersion
-        versionCode = flutter.versionCode()
+        versionCode = flutter.versionCode
-        versionName = flutter.versionName()
+        versionName = flutter.versionName
     }
 
     buildTypes {

examples/layers/android/app/build.gradle

@@ -20,8 +20,8 @@ android {
         applicationId "io.flutter.examples.layers"
         minSdkVersion flutter.minSdkVersion
         targetSdkVersion flutter.targetSdkVersion
-        versionCode flutter.versionCode()
+        versionCode flutter.versionCode
-        versionName flutter.versionName()
+        versionName flutter.versionName
     }
 
     buildTypes {

packages/flutter_tools/gradle/src/main/groovy/flutter.groovy

@@ -78,7 +78,7 @@ class FlutterExtension {
     public String flutterVersionName = null
 
     /** Returns flutterVersionCode as an integer with error handling. */
-    public Integer versionCode() {
+    public Integer getVersionCode() {
         if (flutterVersionCode == null) {
             throw new GradleException("flutterVersionCode must not be null.")
         }
@@ -91,7 +91,7 @@ class FlutterExtension {
     }
 
     /** Returns flutterVersionName with error handling. */
-    public String versionName() {
+    public String getVersionName() {
         if (flutterVersionName == null) {
             throw new GradleException("flutterVersionName must not be null.")
         }