Smooth transition when switching dreaming.

This CL makes the following changes
- instead of stopping any existing dream before starting a new one, now
  stop *after* the new dream has been started
- do not reuse the same task for multiple dream activities

This allows a smooth inter-task transition when swiching between dreams,
e.g. between user configured dream and system dream.

Bug: 244315094
Test: atest DreamControllerTest
Test: atest SystemDreamTest
Test: manually on device, see video in linked bug
Change-Id: I9576470ae0f54d553a0db3aee6fb720edaa5d733