Use a fake coverage module instead of real one.

Actually loading the coverage module was leading to odd concurrency
conditions where it was interfering with mocked calls. This changes
to stub out the coverage class with a fake one so that we don't have
to worry about collisions. It also modifies the coverage class to be
created lazily so it isn't automatically started at import time.
Finally it removes the hack in run_tests.sh ignoring the coverage
tests.

Fixes bug 1131023

Change-Id: I39de434454f8b0903f6311e731e215a93027bc91
Merge "Use a fake coverage module instead of real one."