Optionally trigger CG in onTrimMemory()

Set a trim-level threshold to debug.am.run_gc_trim_level to activate it.

Bug: 135148702
Test: Manual test with "setprop debug.am.run_gc_trim_level 0", run a lot of
heavy apps and take pictures, and check logcat for the "force_gc" event log.
Test: Manual test with "setprop debug.am.run_mallopt_trim_level 0", run a lot of
heavy apps and take pictures, and check logcat for a debug log.

Change-Id: I73b4dc7374e85e9a22c98ab17da53aa6cb25a188