Delete external app dir on app uninstall with FUSE

installd parses /proc/mounts to determine the file path to use when
deleting app dirs on external storage. With FUSE mounts instead, these
/proc/mounts looks very different.

Now, we use the correct mounts on /mnt/pass_through when deleting dirs

Test: atest ExternalStorageHostTest#testExternalStorageClearing
Change-Id: Ic42574a46c4408ce5d43d4f412dce7f9876b8044