Fix USSD Api test app

The test app that sends USSD with the TelephonyManager API was broken and
would never receive callbacks.  Corrected the issues.

Test: Manual
Bug: 37484804
Merged-In: I692b50e497b9aceebe80c87ec6efc25b6046a64b
Change-Id: I692b50e497b9aceebe80c87ec6efc25b6046a64b
(cherry picked from commit 6aab9148)