Fix GattClientTest flakiness by waiting to verify connection first

before disconnecting

This change also reduces the average test time from ~7.5s to ~6.5s as
the wait timeout does not need to happen if mockito can verify the call
quickly earlier

Fix: 357278247
Test: atest GattClientTest
Flag: EXEMPT test only
Change-Id: Ifed9b343098fa403dd391f1ffad107c85a65296e