Silence warning about falling back to checked-in adb binary

There's no need to print out a warning about falling back to the
checked-in adb binary every time telemetry is run. Also turn ADB_PATH
into a memoized function to remove side effects from importing
constants.py. This also ensures the right logging level is set before
the path is determined so the warning is still printed in debug mode.

BUG=329317

Review URL: https://codereview.chromium.org/117333008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241854 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241854