Add support for a circular bitmap overlay for round android wear emulator.

Android wear emulators have never had support for enabling both gpu
emulation and also having a round clip overlay present. Fix this by
supporting a drawable overlay when the ro.emulator.circular system
property is set.

Bug: 13415409

Change-Id: I6e0840ebe5c77adb786a7ba7ec6af348308ca46a