Make sure to sync GPU after deleting memory in trimMemory.

On GL some devices may not delete gpu memory immediately after calling
delete and instead will just queue it up. We must call glFinish to
make sure the memory is deleted when we want it.

More importantly Vulkan memory will not be freed until the GPU is done
with all the work. Thus we require an additional flush to make sure the
previous gpu work we submitted has finished and we can delete the
memory.

Test: manual testing on device
Bug: b/130440975
Change-Id: I2a9202dd7d15d97b365120a8caf9e10b4100efc3