Tearing down the test safely.

If the test fails to initialize properly, tearDown is still called,
and we may have null Launcher activity.

We want to safely skip using that activity to avoid a NullPointerException
that would hide the original error message.

Bug: 303070230
Test: presubmit
Flag: N/A
Change-Id: I3bb07fa55e5793c73caaaeda57168aa11c85893e