Make getSystemService(NFC_SERVICE) the NFC entry point.

This gives NFC service a handle to the application context.

Deprecate NfcAdapter.getDefaultAdapter(), it does not provide a context.
Using this method will print a warning, and will later throw an exception
if a method that requires a context is called. No 2.3 API's will fail, but
new API's that do require a context might fail.

Also add helper NfcAdapter.getDefaultAdapter(Context).

Change-Id: I9a6378de4ef4b61ad922f8d53e64e2a1a1d5d60c