libagl: Switch from gralloc to GraphicBufferMapper

Removes all direct references to the gralloc module from libagl and
uses the GraphicBufferMapper class instead when locking and unlocking
buffers.

Also a couple of minor code cleanups to eliminate warnings.

Change-Id: Ie982d375b3152d5f677ab54c2067179b8d34c06d
5 files changed