Move garbage collect logic from rule to gc helper

Move GC logic from GarbageCollectRule to a helper so that it can be
reused in multiple locations. GarbageCollectionHelper can then be
used to implement device listeners that can GC before metric
collectors.

Bug: 119048359
Test: atest
CollectorsHelperTest:com.android.helpers.tests.GarbageCollectionHelperTest
atest GarbageCollectRuleTest
Change-Id: Ib8c26afcc18d80fc8e71a8c3e2ac59bd0bac4531