Fix a NPE in WearableSensingManagerIsolatedServiceTest

Because the test is skipped for Wear and TV in the @Before method,
the WearableSensingManager is null. So, we should avoid interacting
with it in the @After method to avoid crashes.

Bug: 376534487
Test: atest WearableSensingManagerIsolatedServiceTest
Flag: TEST_ONLY
Change-Id: I7308e27717f0d873c6d1c489e5145aaabd4fa1d7