## Description
This PR adds the 'Share' button to the text selection toolbar on Android.
## Related Issue
Fixes https://github.com/flutter/flutter/issues/138728
## Tests
Refactor a lot of existing tests in order to:
- make them more readable (avoid duplication by introducing helper functions, specify explictly check which buttons are expected).
- make them more accurate (check that expected buttons are visible instead of just checking the number of buttons).
For instance, previous tests contained sections such as:
```dart
// Collapsed toolbar shows 3 buttons.
expect(
find.byType(CupertinoButton),
isContextMenuProvidedByPlatform ? findsNothing : isTargetPlatformIOS ? findsNWidgets(6) : findsNWidgets(3)
);
```
Where the comment is obsolete, the two cases (6 widgets and 3 widgets) are not explicit (which buttons are expected?), and not accurate (will pass if the number of buttons is right but the buttons are the wrong ones).
