HLS: misc changes in LiveSession buffering logic

remove unnecessary time discontinuity

move fetcher to separate looper so that download won't
block LiveSession

poll buffering at 1 sec interval in LiveSession, and
switch bandwidth if necessary

use fixed 100ms threshold for resumeUntil

bug: 19567254
Change-Id: I911e5041364f0858b43f2312756e173db5870a1e