Add PermissionEnforcer class

Defines a new PermissionEnforcer class used to check permission as part
of the @EnforcePermission AIDL annotation. It is registered as a system
service to avoid the cost of creating a new object for each new client.

Bug: 234083358
Test: TH
Change-Id: I2bd23f62a0d207ddac6bbe69666ced96e77ba16f