Add app op reserved specifically for testing

In preparation for using the permission state to compute the app op mode
of runtime permissions it will be needed to have an app op specific for
keeping tests working correctly. In DiscreteAppopsTest the test uses
OPSTR_CAMERA to make noteOps to validate discrete history; the problem
is that the test app doesn't actually request the camera permission so
PermissionPolicyService never syncs the permission state to the appop's
mode which leads to the default mode being used which is the fully
permissive MODE_ALLOWED. When permission grant state is used
synchronously for checking mode this is going to behave differently
since a permission not requested will not be granted.

We will need an app op that is written to discrete registry and can be
granted the fully permissive mode to test background discrete events
and when permission state is used camera will never be able to. The
location app ops are the only qualifying app ops that exist today but we
see the problem that other features on the device also noteOp quickly
after install contaminating expected results and failing the test.
Because of that this can also be seen as a quality and stability
improvement by providing better isolation between the api behaviors we
want to test and other uses on the device of these apis.

Test: Create this constant, update the test, and watch
Bug: 266164193
Change-Id: I90bd030f36a9de4ab63768aedc6ad0b5317d91d0