Keystore 2.0: Revisit GC.

* Limit the number of simultaniously scheduled garbage collection
  requests.
* Reduce the number of database transaction when many keys need to be
  collected.

Test: Ran Cts test for regression testing and performance validation.
Change-Id: I7c1e2321e13f48cb99e83574df3c4178179da633
3 files changed