Remove unused setter

setEnableLowResSnapshots was only added for testing, but it is unused
because we are using a Resource mock to override the value. This is a
better way to test the feature because it ensures expected behavior of
the config overlay.

In addition: remove duplicate test, fix comments for test, and add new
assertions. Also, removed setIsLowResolution in test Builder class to
reflect ActivityManager.TaskSnapshot.Builder.

Test: TaskSnapshotCacheTest
Test: TaskSnapshotControllerTest
Test: TaskSnapshotPersisterLoaderTest
Test: TaskSnapshotSurfaceTest
Bug: 142063079
Change-Id: I83a9503a2f75490e5cf80bdc93041208c4897c6e