Set DreamsActivateOnSleepSetting and handle Sandman in tests

Two PowerManagerServiceTest were failing on TV:
- testRemovedDisplayGroupWakeLock_affectsNoDisplayGroups
- testWakeLock_affectsProperDisplayGroup

These tests expected the device to be DOZING at the end of the test,
but the devices was found to be DREAMING.
This difference came from `mDreamsActivateOnSleepSetting`, which was
not set for phones, but is set on TV.
Two tests are added to test the behaviour of this setting.

This CL also changes a call to `Handler#sendMessage` to
`Handler#sendMessageAtTime`, which schedules the message according to
the time of the test clock, instead of the real SystemClock.
This causes `handleSandman` to actually run during the test,
which would put the device to sleep if DOZING and dream couldn't be
started, so when #startDream is called on the DreamManager mock,
we set #isDreaming on the mock.

Bug: 205837239
Test: atest PowerManagerServiceTest
Change-Id: I6f27ea6a5cf8ddb1b7735c8d6ef67f27a5eb3747