Unmount pass_through path last.

There've been reports of issues where, when a volume is ejected, the
MediaProvider process gets killed. This happens because the
MediaProvider has a file open on the volume (eg, during a scan). We do
abort the scan when the volume is ejected, however this could take some
time. So, we give MediaProvider a bit more time before getting killed,
by only looking for files open on the pass_through paths last. This
order anyway seems to make more sense - ideally we kill apps using
external storage before we unmount the pass_through path underlying it.

Bug: 171367622
Test: atets AdoptableHostTest
Change-Id: Ie8eacaa72a80ff8161ecf1e8c0243afcd890ee39