Check for user consent denial when dumping traces.

VM traces is one of the longer running operations. Check consent
periodically and return early if necessary.

BUG:128270426
Test: manual; by canceling when traces was running.
Test: verified "VM TRACES JUST NOW" shows up as expected
after refactoring in the normal flow as well.

Change-Id: Ic5282b948929857850730d6b06ec0cae8c39a99f