Move listener registration out of constructor.

Work being done in the constructor was resuling in an NPE when a
callback was being called too quickly.

Add an idempotent init method to do all the setup work.

Fixes: 188015288
Test: atest SystemUITests
Change-Id: Idc0f3c7a6a53f447194b60486d814dcd76af9f25
5 files changed