Add an API to manually invoke Android Beam.

Activities can call NfcAdapter.invokeBeam() to
manually invoke the Beam animation. Any NFC tap
is then enough to complete the transaction.

Also, added NdefRecord convenience method for
creating Text records.

Bug: 5134061
Change-Id: Ia9df360d1d7e8451157c85a6d12f2a4eec924960