Add IS_RUNNING_IN_TEST_HARNESS check for isTransientTaskbar

This avoids us trying to read FORCE_PERSISTENT_TASKBAR feature flag
which apparently crashes tests due to permission issues. Going forward,
when we support persistent taskbar better, we should update the tests
and this check as commented in the code.

Test: com.google.android.apps.nexuslauncher.TaplTestsNexus#testSearchNoInputs
Fixes: 261604544
Change-Id: I35c20825e8171766834f0f32be4f3818d02ab1df