Preventing NPE crash if view is detached from window.

View#getDisplay can return null if the view is detached from window.
Get the display by Context#getDisplay should ensure there can always
return the local display object.

Bug: 209920544
Test: enable shell transition, repeatly run AppConfigurationTests
testConfigurationUpdatesWhenRotatingWhileFullscreen and verify no crash
in system ui.

Change-Id: I792417a55a588c9552529f2815c1ab5d88f427e7