[Ongoing Call] Make the uidObserver a val so that we're guaranteed to

not accidentally register it multiple times in a row.

This is a follow-up fix from comments on ag/15503430.

Fixes: 216520671
Test: Verify the call chip still disappears when you're in a call and
you open the call app.
Test: atest OngoingCallControllerTest
Change-Id: I9454b27961d9277a6c85e16c196d6ed0ddc6273e

Change-Id: I8073f178566754037e25af57d2b9949f0d6a396d