Fix ImsManager NPE encountered during GTS testing

ImsManager was throwing a NPE because multiple requests to get
ImsService look to be getting scheduled in a race condition where
we are internally scheduling a retry and the binder thread is
scheduling a retry.

Fix synchronization in retry block and check for null ImsManager
just in case.

Test: atest GtsTelephonyTestCases
Change-Id: I5e4b5bf656642c1d26a88fe77726c44f2cfe8607