Allow multiple instances of DreamActivity to be launched

In some cases, we may decide to switch between the different dreams in
the same package. During the switching animation, the package may have
two dreams active at a single time. This change allows that to happen,
maintaining a 1:1 mapping of DreamActivity <-> DreamService.
DreamManagerService is then responsible for handling the lifecycle of
the activities, by stopping the previous dreams once we've fully
transitioned over to the new dream.

Fixes: 324063099
Flag: ACONFIG android.service.controls.flags.home_panel_dream TEAMFOOD
Test: flashed and verified switching between home panel and lowlight
dreams now work

Change-Id: I3abcb186571d83dd0d85e6dbe93fe25b21611b40