HIDL: add APIs used by subclasses to SystemApi

Two methods:
- HwBinder()
- HwBinder registerService
- HwBinder onTransact

are used by Stub classes generated by hidl-gen.

Bug: 72480743
Test: build hidl-gen generated stubs with system_current
    after manually copying stub jar to prebuilts.
Change-Id: I9b409271b683ad4769a0420e1688b14c561d39d1