Delay Explicit GC from the SystemServer.

Binder activity as part of the System Server executes an Explicit
GC every 200 Binder references. Such Binder activity is significant
during the application launch of some Google Apps like Music.
The resulting pause due to the GC pauses the main application, affecting
launch time.

This change detects an app launch, and delays the incoming GC by
3 seconds. The API for GC delay can be calllable for different use-cases.
This change deals with only the application launch.

CRs-Fixed: 779174

Change-Id: I0f2dfaf3cb13940c77df8a6fefc2cbc0c76e027c