15153 Refactor BitmapCanvas, lazily allocate canvas, fix image composition bug. (cla: yes)

15154 Bump simulator version in IosUnitTests & scenario app in preparation for luci xcode 11 migration (cla: yes)

15156 Add a deprecation javadoc note to the old FlutterActivity (cla: yes)

15160 Recover when browser throws on ImageElement.decode due to too many images (cla: yes)

15161 Add .vpython dependencies to migrate run_tests.py to vpython. (cla: yes, waiting for tree to go green)

15164 Reduce default verbosity of Android embedding (affects: add-to-app, cla: yes, waiting for tree to go green)