Handle teardown after incomplete setup in locksettings tests

In JUnit, @After methods are called even if a @Before method throws an
exception.  Therefore @After methods need to handle the case where
objects created by @Before have not been created yet.

This only makes a difference when there is already a test failure, but
this prevents a misleading error message.

Bug: 396780038
Test: atest FrameworksServicesTests:com.android.server.locksettings
Change-Id: Ia07bca84849dfe7fac54e89abdba2cfb5a823cae