Fix issue #23037093: "Write system settings" permission value fails...

...to persist after reboot

We were writing a corrupt settings file, so would always reset back
to the default app ops state after boot...!

Also add new appops service commands to manually write and read
its settings, since that is very useful for testing.

Change-Id: Ia510507764738fd82e45ec0be6db840c6ea30c28