Do not block gestures when dream is not active.

This changelist short circuits logic checking whether a dream should
block back gestures. Subsequent logic checks whether a dream is in
preview mode, which is only valid if a dream is running.

Test: atest DreamOverlayServiceTest#
Fixes: 364510142
Flag: EXEMPT bugfix
Change-Id: Ibf3cbfa0834d99a770501c16785cda5fdbdb034f