Logging for Nexus home activity appearing after to-home gesture in L3.

Utilities.IS_RUNNING_IN_TEST_HARNESS is used as a condition
for logging in the background NexusLauncher where sDebugTracing is
not set.

Bug: 192018189
Test: presubmit
Change-Id: Ib08656a12c778b7bf968532e3ba899a03031fd7b