Disabling the activate button when paused

This revises the previous attempt to drop taps when another window is
obscuring the device admin details. Any system alert windows or toasts
are disabled when the activity is in focus, otherwise the button is
disabled so no floating activities can obscure it and delegate taps back
to the security sensitive "Activate" button.

Test: CtsVerifier -> Device Administration -> Device Admin Tapjacking Test

Bug: 35056974
Change-Id: I292a44e713ea4a4b0305ca3b3b21f1c82fd73c2b
Merged-In: I292a44e713ea4a4b0305ca3b3b21f1c82fd73c2b
(cherry picked from commit af5d8b9b)