Dispose GsmCdmaConnection object in teardown.

Noticed this log line in some unit test bugreport:
am_wtf  : [0,13185,com.android.frameworks.telephonytests,545832516,
PowerManager,WakeLock finalized while still held: GsmCdmaConnection]

Test: atest FrameworksTelephonyTests
Bug: 169117139
Change-Id: I5cb7ae9d68e46e302e0825ba5ee1d9218c9fa32c