Determine pass_through mount mode based on UID.

We previously using the packagename, but if we start the media process
for another package (eg DownloadProvider, which is hosted in the same
process), then the packagename won't match, and we would mount the FUSE
mount instead.

Bug: 13789012
Test: atest AdoptableHostTest
Change-Id: I9b5fea07f3620b28595274a264f96d9f891f6715