Restrict MANAGE_DEVICE_ADMIN permissions to signature

We want to prevent privileged apps from calling DevicePolicyManager#setActiveAdmin. After this, only apps signed with the platform certificate (e.g Settings / ManagedProvisioning) will be allowed to set Device Admins.

Leave @SystemApi so that it is clear that it 'was' a privileged permission, but add @removed to show that it is no longer a privileged permission.

Bug: 111153365
Test: atest PermissionsHostTest && atest CtsAdminTestCases
Change-Id: I688e87cf09ad206f4f517a7be960c2aa01af8fc4