DO NOT MERGE: Prevent non-system IME from becoming device admin

Currently selected IME can inject KeyEvent on DeviceAdminAdd screen to
activate itself as device admin and cause various DoS attacks.

This CL ensures KeyEvent on "Activate" button can only come from system
apps.

Bug: 280793427
Test: atest DeviceAdminActivationTest
(cherry picked from commit 70a501d0)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:0976cd789d3bfb593e73237b5b0adc39933a1c1c)
Merged-In: I6470d1684d707f4b1e86f8b456be0b4e0af5f188
Change-Id: I6470d1684d707f4b1e86f8b456be0b4e0af5f188