Add end-to-end tests for @EnforcePermission

The current integration tests under system/tools/aidl mock
PermissionEnforcer and as such do not exercise the complete permission
checks. Set up a service app (service-app) which relies on the
annotation to protect its methods. A nested service is included to test
that permission checks propagate to same-process calls.

Bug: 269721152
Test: atest EnforcePermissionTests
Change-Id: I2d14f10333eabc385007216038dc487f83a4d21a