Closes https://github.com/flutter/flutter/issues/106416.
This PR adds a new `flutter config` setting named `jdk-dir`. When set, the tool will use the JDK found at this location for all Java-dependent tool operations such as building Android apps via gradle and running Android SDK tools.
Fixes#124252, finishing work on the umbrella tracking issue, #126126.
Essentially, after this PR, no (non-test) code should be be referencing/invoking the java home or binary paths.
Reverts flutter/flutter#126086.
This PR changed the interfaces of some classes, namely `AndroidSdk`, and deleted a global.
These classes had custom overrides in g3 that were not updated with a g3fix, so this PR has broken tests. See https://b.corp.google.com/issues/281945232 (non-public link)
This is the first step in unifying Java-finding logic across the tool. If curious, see #126029 for an example of what all the changes will probably entail.
Moves java-related functionality like `AndroidSdk.findJavaHome` to a new class, `Java`.
See tracking issue https://github.com/flutter/flutter/issues/126126 for more.
