Updating logcat reading logic

Start a thread to read logcat synchronusly instead
of back-tracking at the end of the test

Also:
* Reusing the same logcat process for all checks. This eliminates
reading the log from its start for every gesture. We don’t kill that
process at the end, and don’t stop the thread. I’ve verified that
“am instrument” doesn’t hang, and the logcat process gets automatically
killed after the test process exits.
* Not using mStarted latch, as there is no need to wait until the reader
reaches the start mark.

Bug: 149422395
Change-Id: Ide4ed19ad8d099c41918f38c2b073b8b2e143b69