Fix StatsEvent memory usage for pulled events

Add usePooledBuffer flag to the Builder which determines whether to reuse
the Buffer's byte array in StatsEvent or use a copy.
The build() function also calls release() on the Buffer
if a copy of the Buffer's byte array is passed to StatsEvent.

Also, for pushed events, release the StatsEvent object and consequently,
the Buffer in StatsLog.write(StatsEvent)

Fixes: 145026572
Fixes: 144126444
Test: bit FrameworksCoreTests:android.util.StatsEventTest
Change-Id: I1cdaf0027b69281cb7cb6f3c8ca923d03829b4dd