Added guard to logging in AsyncTask

An async task was trying to get the context
in an InstrumentedFragment. This could return
null if the context disappears before the new
thread gets to that line. Logging will now
be aborted if context is null to avoid this.

Test: Manual/Monkey Stability Test
Bug: 31058928
Change-Id: Iae97d389e58a76b0f213a2749415501894ebf649
(cherry picked from commit 566b66e2)