Add Share button to the SelectableRegion toolbar on Android (#141447)

## Description

This PR adds the share button to text selection toolbar buttons on Android ~~and iOS~~ for `SelectableRegion` (and therefore `SelectionArea`).

https://github.com/flutter/flutter/pull/139479 adds this button for `EditableText` (which is used by `TextField` and `SelectableText` but not by `SelectionArea`).

**Edit**: supporting this on iOS will need more work (see https://github.com/flutter/flutter/pull/141447#issuecomment-1889942622 and https://github.com/flutter/flutter/issues/141775).

## Related Issue

Follow up for https://github.com/flutter/flutter/issues/138728

## Tests

Adds 1 test.
https://dart.googlesource.com/external/github.com/flutter/flutter/+/bef97630088495c31f0b6dd062771401803a94c8

DEPS

diff --git a/DEPS b/DEPS
index eccdb81..afa09c7 100644
--- a/DEPS
+++ b/DEPS
@@ -277,7 +277,7 @@
 
   'engine/src/third_party/dart': Var('dart_git') + '/sdk' + '@' + '1bd8755035ee9c4ac5600a7304f5c8f1c38f82ac',
 
-  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '63018fe6cbd7f95a0e3d3e64b488ccb660430d55',
+  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + 'bef97630088495c31f0b6dd062771401803a94c8',
 
   'engine/src/flutter/third_party/rapidjson':
    Var('flutter_git') + '/third_party/rapidjson' + '@' + 'ef3564c5c8824989393b87df25355baf35ff544b',

commits.json

diff --git a/commits.json b/commits.json
index ef13786..3cc441c 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "engine/src/flutter":"fd0335a910b8ed29ce92e209f25bf03ae267f5be",
   "engine/src/third_party/dart":"1bd8755035ee9c4ac5600a7304f5c8f1c38f82ac",
-  "flutter":"63018fe6cbd7f95a0e3d3e64b488ccb660430d55"
+  "flutter":"bef97630088495c31f0b6dd062771401803a94c8"
 }
\ No newline at end of file