Address race condition in procstate time tracking

The issue occurs when the application proc state changes
rapidly, e.g. from FOREGROUND to BACKGROUND to CACHED.
The original code would sometimes attribute the time slice
to the wrong proc state.

Bug: 192550308
Test: (on cuttlefish) atest --rerun-until-failure 300 FrameworksCoreTests:com.android.internal.os.BstatsCpuTimesValidationTest -- --abi x86_64

Change-Id: Ic22bbfa3aae701014fc016ec9e2d32b1c528d462