L API proposal: Introduce IS_RTL flag

This CL introduces CursorAnchorInfo.FLAG_IS_RTL for better
RTL support. This CL also renames *CharacterRect() with
*CharacterBounds() so that they can look more consistent
with other existing APIs.

Rationale:

CursorAnchorInfo.FLAG_IS_RTL addresses following issues.
1. There is no way to associate the RTL information with
   the insertion marker.
2. Returning mirrored (right < left) RectF for RTL in
   CursorAnchorInfo#getCharacterRect() is turned out
   to be bug-prone. Such usage of RectF is not fully
   supported. For example, RectF#isEmpty() always returns
   false when right < left.
3. There is no reliable to provide the RTL information
   when CursorAnchorInfo#getCharacterRect() returns an
   empty (right == left) RectF. Perhaps we could use +0.0
   and -0.0, but I'm afraid that it is also bug-prone.

BUG: 17365414
BUG: 17335734
Change-Id: Ic8c6fab58c01206872a34e7ee604cdda1581364d