[ActivityManager] Fix index out of bounds when updating next pss time.

Symptom:
System server crash.

Root Cause:
The value curProcState for array index is -1 if the process
has not attached yet.

Solution:
Skip computing for process which is not attached or curProcState
is nonexistent state.

Change-Id: I71aaf45bb78d73097ebe9dfebf76b72f2d243232