Rename full and reduced scale configs

Rename:
  config_fullTaskSnapshotScale -> config_highResTaskSnapshotScale
  config_reducedTaskSnapshotScale -> config_lowResTaskSnapshotScale

Both full and reduced scale can be "reduced" from 100%, so name them
more clearly.

Test: TaskSnapshotCacheTest
Test: TaskSnapshotControllerTest
Test: TaskSnapshotPersisterLoaderTest
Test: TaskSnapshotSurfaceTest
Bug: 148617404
Bug: 142063079
Change-Id: Ie8073d5a3048c19450308b2af22d363deaa51b6a