Make ControlsActivity translucent

This has the (for now) desirable side-effect of preventing
animations when opening/closing. Being a frame around TaskView
the normal close animations don't work for this Activity.

Once the Activity isn't a TaskView (b/293338460) this can be
reverted.

Bug: 312649396
Bug: 293338460
Change-Id: Iad2efcb727b97c923a4608e5fae9ad722cc412f8
Test: Manual - open/close over home and lockscreen