Add config to enable snapshot preloading

Enable preloading if config_enableTaskSnapshotPreloading instead of
assuming it should be off for all low ram devices. This allows it to be
configured via an overlay.

Test: TaskSnapshotCacheTest
Test: TaskSnapshotControllerTest
Test: TaskSnapshotPersisterLoaderTest
Test: TaskSnapshotSurfaceTest
Bug: 148099851
Change-Id: I9e3320dcff5b710e110cc7199c41c5a004991592