Fixes test process crash caused by unsynchronized I/O handling.

Flush all remaining items in the persist queue before cleaning up the
test directory.

Flag: EXEMPT bugfix
Bug: 420172484
Test: atest ActivitySnapshotControllerTests TaskSnapshotCacheTest
TaskSnapshotLowResDisabledTest TaskSnapshotPersisterLoaderTest

Change-Id: I38fce6e0effcacb0b595a589cf9573cae58a4586