Remove NFC handling from AbstractMailActivity

1. Remove the NFC code left behind after
https://googleplex-android-review.googlesource.com/#/c/210327/

2. Expose one fewer method from MailActivity.

3. Does not fix the activity leak, but does make leaks easier to
   debug.

Bug: 7883592 mild activity leak

Change-Id: Icde61c9b3251bf1686c50e4b5184d3d56fd2941a
4 files changed