Delay handler messages until class is fully initialized

KeyguardViewMediator must wait for the both the #start() method on its
own class, as well as that of CentralSurfaces, since CentralSurfaces
needs to call #registerCentralSurfaces to complete the class
setup. Before this fix, the delay could cause an NPE when SystemUI was
restarted.

If this situation is detected, repost the same message but with a
small delay in order to check again.

Fixes: 319540806
Test: atest KeyguardViewMediatorTest
Flag: N/A
Change-Id: Ib821ff3742ac9bce67d05b6b29ae1ea9fcd42c5a