Adds a check for the case where a device has no battery present. The
devicelab test expects the text "No battery." in the case where a device
has no battery. This is triggered by returning an error with a
NO_BATTERY error code.
Also applies some cosmetic fixes:
* Amends a copyright notice to Flutter instead of com.example
* Deletes an unused test.
Issue: https://github.com/flutter/flutter/issues/79204
Adds a macOS implementation of the platform_channel example,
demonstrating method channels and event channels with a battery power
plugin.
Adds platform_channel_sample_test_macos macOS host test to verify the
sample works.
Issue: https://github.com/flutter/flutter/issues/79204
