Cleanup android.text.TextUtils a little.

- Clarify that getTrimmedLength() only considers ASCII control
  characters as control characters.

- Deprecate getReverse() instead of trying to fix it, as it's really
  not meaningful for internationalized text.

- Support non-BMP characters in isDigitsOnly().

Change-Id: I947c449b48c252ecc7f7299145f6f8fbad86004f