Allow activities to be embedded within the dream

When our flag is enabled, the dream activity will be launched with
SINGLE_TASK instead of SINGLE_INSTANCE. This allows other activities to
be launched in the same task as the dream. The dream will also be
resizable, as this is a requirement for activity embedding.

Bug: 298025023
Test: verified on device that embedding activity within dream works
Test: atest CtsWindowManagerDeviceOther:DreamManagerServiceTests on cf_pc
Flag: ACONFIG FLAG_HOME_PANEL_DREAM DEVELOPMENT
Change-Id: I4b17dd78fbeff3f4ddd235934d825b05f870b9b4