Fix brief flash when making a call from search

Perform the fragment transaction to hide search in onPause.
This prevents the brief flash that shows up in the time between
pausing the activity and starting the in-call UI, but also
ensures that we execute the transaction before onSaveInstance is called
to prevent b/10953115.

Bug: 10780429
Change-Id: I85b4c7e6719acede845d604087344401edc9ef3d