Revert "Revert "binder: Prevent context manager from incrementing ref 0""

This reverts commit f0416df7.

Reason for revert: This was a "temporary" reversion to workaround what is believed to be a user-space issue.

Change-Id: I5322aecfe57cd8237e6657525eb33975c4840059
Bug: 166779391
Signed-off-by: Todd Kjos <tkjos@google.com>