Disable dismissing the dream on activity start

This is causing dreams to be terminated when activities start in the
background without any user activity. This makes it appear as if the
dream is randomly stopping.

Instead, activities may set the `turnScreenOn` flag if they wish to
launch over the dream.

Bug: 198041170
Test: atest DreamManagerServiceTests
Test: manually on device
Change-Id: I256b543de2ff07a91c451871db61b3ca516db79d