* Add support for software text editing controls

Includes selection, copy, cut, paste, as well as partial support for up
and down movement.

Text editing controls can be accessed in GBoard by:
  top-left arrow > three dots menu > text editing

Partial fix for flutter/flutter#9419 and flutter/flutter#37371.

* Introduce InputConnectionAdaptor tests

Run with:
  testing/run_tests.py --type=java --java-filter=io.flutter.plugin.editing.InputConnectionAdaptorTest

* Fix BUILD.gn comment on run_tests.py --java-filter flag