GpuStats: Cache the gpu service binder

Cache the gpu service binder in the app process so that sending stats pieces
won't try to find gpu service every time. If gpu service accidentally restarts,
just ignore the later stats from the app, since the memory that holds GpuStats
for that app is gone already during the gpu service restart.

Test: systrace while try killing gpu service
Change-Id: Ib9c38a3592f509f18d146b6c6e8f14e9e997c994