Fix issue with call backs from media process.

All but a few lines of this is for issue #16013164, which allowed
apps to do some operations as the media uid by having it call
back to them to open a file.  The problem here is with the tempory
identity stuff in the activity manager, allowing us to make the open
call as the original caller...  ideally we should figure out a way
to just get rid of all of that, but the solution here is actually
easier (even though it doesn't look it) -- we now hand a token over
to the openFile() call that it can use when doing permission checks
to say "yes I would like the check to be against whoever is responsible
for the open".  This allows us to do the uid remapping for only this
one specific set of permission checks, and nothing else.

Also fix issue #17487348: Isolated services can access system services
they shouldn't be able to.  Don't send any system service IBinder objects
down for the first initialization of an isolated process.

Change-Id: I3c70e16e0899d7eef0bae458e83958b41ed2b75e