atrace: Don't NPE if we can't access hal-related tracepoint files.

This currently NPEs when Traceur attempts to start a trace.
Based on b/34242478, this is currently expected and we should fail
gracefully instead.

Bug: 37281960

Test: Starting a trace using Traceur on a userdebug build succeeds.

Change-Id: I082d5750625dec0e1db4a1dd341dc332d8dd752f