powerstats: Clients of PowerStatsInternal must nullcheck

Added comments and annotations to the PowerStatsInternal and
PowerStatsHALWrapper classes. Also fixed the native code to return
partial results if available.

Bug: 179684579
Test: On battery I no longer see crash
Change-Id: Ic97fb56af6559a3fa65f857c527b10303238ac42
5 files changed