GpuStats: refactor single stats pieces into a uniform way

This change makes it easy for adding single stats pieces into GpuService without
adding or modifying the binder interface each time.

Bug: 141003796
Test: adb shell dumpsys gpu
Change-Id: I2907065a55d03a6c1494737e6f0a77f6e94272eb
Merged-In: I2907065a55d03a6c1494737e6f0a77f6e94272eb
9 files changed