Access mock location is no longer a runtime permission - settings

The access mock location is no longer a runtime permission. It is a
signature protected one that apps cannot get but the fact they request
it means they want to inject location into the system. Now the user
gets to choose the current mock location app in developer options from
the apps that request the mock location permission. The access to mock
location is no longer guarded by the permisson but from a new app op
which is off by default and the settiings UI sets it to enabled only
for the currently selected mock location app.

bug:21078873

Change-Id: I6555179ecf0cc37d9bb857e9dfb3b04c091ea612