Verify arguments passed are non-null.

We should verify they are non-null before
handing them off to other threads to avoid
crashing the system_server process.

Bug: 356731520
Test: atest services/tests/powerstatstests/src/com/android/server/powerstats/PowerStatsServiceTest.java
Flag: com.android.server.powerstats.verify_non_null_arguments
Change-Id: I5b0cbf86976e74fd01e3d70ac01a03398ecdaa56