Hiding app shelf if keyguard is occluded - the shared part for 'master'.

Look at the bug for the repro.

In this scenario, new user's setup program starts as a window with
FLAG_SHOW_WHEN_LOCKED flag, which causes 'occlusion' of the keyguard,
i.e. we simply remove it, before switching to the new user.
So, the user sees the old user's shelf with all its apps.

The fix hides the shelf while the keyguard is in an occluded state.

Bug: 28053346
Change-Id: Iedea22a114eafd2c18837a006e4eabee0fe0dd60