Enforce 1-1 relationship between context and NfcAdapterExtras.

This fixes a bug where NfcExecutionEnvironment.close() would NPE
if you called it on a different EE to the one you opened. We now
always return the same EE in the same contet.

Change-Id: I949998dc2ee738527ee281cae46ae50ea6950a2c
1 file changed