The flush and submit at the start of trim memory now syncs the
cpu so that all work is done on the gpu and the max possible
amount of resources can be freed

This fixes CacheManager unit test since the submit at the end
trim memory was causing new resources to be available to be
purged.

Test: locally ran unit tests
Bug: 178978639
Change-Id: I8b57b042dd146fc6c1083540715295e93fd113d3
(cherry picked from commit 846644a4)