Installers will be allowed to write OBB for any application; this is not
easy to achieve on sdcardfs, where the GID of Android/obb is the same as
the GID of Android/data (app-private data), meaning giving installers
write access to Android/obb would also give them write access to
Android/data.
Instead, we create a /mnt/installer view, which is exactly the same as
/mnt/user, with the sole exception that the write sdcardfs view of
Android/obb is mounted on top. This is what will allow installers to
write there, while still being restricted with respect to app-private
data in Android/data.
Bug: 134706060
Test: atest AdoptableHostTest
Change-Id: If2b93870a877efef182bdc06466552a7527499ad
3a2dbfee88