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Introduce
  setNdefPushMessage()
  setNdefPushMessageCallback()
  setNdefPushCompleteCallback()

Deprecate public API
  enableForegroundNdefPush()
  disableForegroundNdefPush()

Hide & Deprecate staged (public but never released) API
  enableForegroundNdefPushCallback()

The new API's do not require the application to explicitly call
enable()/disable() in onPause()/onResume(), we use a Fragment behind
the scenes to manager this automatically.

NDEF Push can be disabled by using a null parameter, so each
enable()/disable() pair is collapsed to a single set() call.

Application code should now look something like:

    public void onCreate() {
        NfcAdapter adapter = NfcAdapter.getDefaultAdapter(this);
        if (adapter != null) {  // check that NFC is available on this device
            adapter.setNdefPushMessage(myNdefMessage, this);
        }
    }

And that's it - no need to explicitly hook into onPause() and onResume() events.

Also - introduce a generic NfcEvent class that is provided as a parameter on
all NFC callbacks. Right now it just provides the NfcAdapter, but using
the wrapper classes allows us to add more fields later without changing
the callback signature. (i'm thinking Bluetooth).

Change-Id: I371dcb026b535b8199225c1262eca64ce644458a