Make Keyguard window focusable so that adb shell input command registers

This change allows existing CTS tests to work properly by entering
test passwords programmatically. This is also a requirement for support-
ing back button pressed event handling.

Bug: 154829257
Test: Manual & CTS tests (atest CtsWindowManagerDeviceTestCases:KeyguardLockedTests)
Change-Id: I13db0153b792f4ef6941067dc1aa06793f033116