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
Test: Manual - open/close over home and lockscreen
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:67b8fa60df8a5a58973167fff7ac2bddbb8995b7)
Merged-In: Iad2efcb727b97c923a4608e5fae9ad722cc412f8
Change-Id: Iad2efcb727b97c923a4608e5fae9ad722cc412f8