Allow HIDL to be used in priv-apps.

Marking HIDL as SystemApi to allow vendor apps to talk to their own
native HIDL services. This includes implementing HIDL callback objects.

HIDL (HAL interface design language) is a language that is used to
facilitate vendor<->system communication which provides guarantees
of ABI stability. For system and vendor services alike, this can also
be used for internal communication.

Bug: 67725182
Test: boot device, hidl_test_java
Change-Id: I9d028cc43ce6f81ea619f01e18f1cd772e17cbfa