Hide system UI as appropriate when the window focus changes.

This follows guidance as given in
https://developer.android.com/training/system-ui/immersive#java



Bug: 141137205
Test: Check sysUI visibility on window focus in SetupWizard
Change-Id: Ic9cf95eb8bd20488e6e024525a1519cec178e246
Merged-In: Ic9cf95eb8bd20488e6e024525a1519cec178e246
Signed-off-by: Julius D'souza <jdsouza@google.com>
(cherry picked from commit ceae6beb0079ec9d38a465650c67503d49392dc7)