Restore auto-launch behavior for ACTION_CHOOSER with a single choice

For the initial release of Marshmallow auto-launching was suppressed
for ChooserActivity if there was only a single choice in order to let
the user confirm what would be launched. In practice, many apps use
ACTION_CHOOSER when they should probably use implicit intents, but
still others have use cases where setting a default doesn't make sense
and the user should still be able to make a choice when one is
available.

As the user confirmation didn't buy much in terms of developer API
expectations (ACTION_CHOOSER being a forced choice) and it adds
speedbumps to existing apps in the ecosystem, revert this change.

Bug 27243827

Change-Id: Id8fd5385d5b1f459e80b0096efe7e2944264739a