Remove static activity instance

1. A static activity reference is too unwieldy; remove the static
   activity object in favor of a static string object.

2. Do less work when changing accounts: instead of creating a new NDEF
   message every time an account is changed, we store away just the
   account name, this is significantly cheaper.

3. Construct an NDEF message on NFC push requests, since NFC events
   are a lot less frequent than account changes.

4. Minor changes to remove unused variables, and removing incorrect
   comments.

Bug: 7883592 mild activity leak

Change-Id: I88f634ab79fe7785cf8a89397d211ea7736c5edb
1 file changed