nfc(api): Create privileged setReaderMode API

Since NFC is becoming a mainline module, the existing CameraServiceProxy
usage of raw INfcAdapter aidl interface will not be mainline compatible.
So, create a system api to formalize this API usage. This has also been
requested by the DCK team previously.

Also, preserve the existing INfcAdapter usage behind the flag by
including INfcAdapter file directly inside services/core.

Bug: 303286040
Bug: 290843937
Test: atest CtsNfcTestCases
Change-Id: I09b9bc58bd4f7cea705a0a17268448dde2bfef0a