Limit ComplicationHostViewController observer to attached lifecycle.

This changelist makes the ComplicationHostViewController only observe
the lifecycle state while the associated view is attached. Previously,
this observation started during init and was never unregistered.

Fixes: 378102090
Test: atest ComplicationHostViewControllerTest
Flag: EXEMPT bugfix
Change-Id: I5603da2b5dd6e6b482b67e982458d2fa865b7259