libbinder: ParcelableHolder - avoid utf-8<->utf-16

Switching getParcelableDescriptor to String16:
- consistent w/ interfaces
- can use StaticString16 (no static dirty heap allocation)
- avoid converting these strings utf-8/utf-16

Bug: 170909836
Test: aidl_integration_test
Change-Id: I51f81fc89d95f2697524512b9d5cf3bd669dbb0e