Fixed keyguard inflation problem

Inflating a keyguard while dettached from window would
raise a NullPointerException since window insets wouldn't
be available at the time.

Test: packages/SystemUI/tests/src/com/android/systemui/statusbar/phone/KeyguardBouncerTest.java
Change-Id: I7f48a62807877537814e6f34db1e6fa6fbec7923
Fixes: 65495303