Add get_accounts app op

For each runtime permission we have an app op to toggle the
permission for legacy apps as they cannot handle permission
revocations. We were lacking an app op for get_accounts
which prevented the user from controlling access to accounts
regardelss that they change the state of the permission
toggle in the UI. Even worse the permission UI is written
with the assumption that every runtime permission has an
app op and as a result revoking the contacts group (if the
app requests the get_accounts permission) is reset back to
allowed in the UI.

bug:23854618

Change-Id: I9e3f9bfeb320bed561d718db99ee285915d5701b