[Scribe] Support extended handwriting boundary

To improve the user experience the handwriting area can be larger than EditText's view boundary.
This CL extended the size of each EditText's handwriting area by 20dip.

Bug: 211764956
Test: atest FrameworksCoreTests:android.view.stylus.HandwritingInitiatorTest
Test: atest android.view.inputmethod.cts.StylusHandwritingTest
Change-Id: I5980be2f6ae28ee13d486747fe8a371b1b5f82e6