Test for writing and loading reduced resolution task snapshots

Added a test to write and load only low resolution task snapshots if the
snapshot's flag of reducedResolution is set to true. Then verify only
the low resolution bitmap is saved and not the full sized. This is for
the change ag/2579729.

Test: runtest frameworks-services -c
com.android.server.wm.TaskSnapshotPersisterLoaderTest
Bug: 63940837
Change-Id: I685046bdac0cddb9e24271152c91e7912c595e7f