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
Change-Id: I6470d1684d707f4b1e86f8b456be0b4e0af5f188
(cherry picked from commit 70a501d0)