Only hide sensitive content when Keyguard is showing

isOccluded is set delayed when unlocking the phone from a
notification, so updatePublicMode used a stale state. Now, only
the fact whether the Keyguard is showing (no matter if occluded or
not) is used to determine the public mode, which is set in a
synchronous manner.

Bug: 18026898
Change-Id: I3e4988f3e806cf15616ee5aa452f81f19d4aa716