Trim the heap when idle after concurrent GC's.
Bug: 4984352

When applied system-wide, trimming the heap can help recover
multiple megabytes worth of pages that are no longer used.
This is especially useful for applications that periodically
allocate large objects during initialization that they then
discard when running in the background.

Change-Id: Iafffd65a70ae62022f0c74f836a84a7afbe23c3c
3 files changed