Refactor AccessProvider and ScopedAccessProvider

Refactor AccessProvider and ScopedAccessProvider

AccessProvider is a helper class for setting up Smack rules, user id,
group id and process labels before or during a test. CKM tests also
contain different AccessProvider and ScopedAccessProvider classes, but
only use a single constructor of the latter to pretend to be an app.
These classes contain some duplicated code. Also, after the removal of
libsmack-tests, the responsibilities of these classes have shrunk to
pretending to be an app and nothing else.

I have cleaned up src/common/ AccessProvider, renamed it to AppContext
and made it flexible enough so that ScopedAccessProvider can be
implemented in terms of it and src/ckm/ AccessProvider can be removed. I
have then cleaned up ScopedAccessProvider and renamed it to
ScopedAppContext.

Change-Id: I325f7bd1d9c2ac276960530384682227cefec7da