Check for null application context in UserManager.

UserManager currently always uses getApplicationContext for its
internal context object, but that method can return null under certain
circumstances. This CL adds a null check, so that the original context
is used if the app context is null.

Bug: 232179155
Bug: 233915566
Test: atest UserManagerTest
Test: install Apex Launcher and App Hider on device with work profile
Change-Id: I26bcf693da8f5bc53126f79f9e6ffb17bd87fc84