Revert "Fix the memory leak in BatteryStatsImpl.Timebase.mObservers."

This reverts commit 420ed9ef.

Reason for revert: The WeakReference change can prevent memory leak, but 
but the extra time spend on creating thousands of WeakReference objects has slowed down system_server startup as reported by b/111658334. Revert this CL for now, will find other approach to prevent memory leak. 

Change-Id: I07a5806e36159467459b841f63e2b0c55b6cc10b