Implement Binder shell command for settings provider.

The provider now published a system service, through which you
can do direct shell commands.  The commands are a copy of what
the existing "settings" command does (in a follow-up, that will
be converted to a simple script that calls this).

Also improved a few things in the provider:

- Don't allow implicit creation of settings data for users that
don't exist.
- Improve dump to include the package name that applied each setting
and remove misleading stuff from historical data (and this is now
available through "dumpsys settings" instead of the provider dump).

Test: manual

Change-Id: Id9aaeddc76cca4629d04cbdcbba6a311e942dfa6