[MediaFling] Start a fresh session every time

When flinging videos, we should start a fresh session every time so that
the session state is cleaned up properly and we don't control previous
flinged videos.

Bug: 711860
Change-Id: I6be7e993f60294276e919cf4b2b7f5d19d176b07
Reviewed-on: https://chromium-review.googlesource.com/c/1371012
Commit-Queue: Zhiqiang Zhang <zqzhang@chromium.org>
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616785}