Make PendingIntent.send public API.

There are several use cases where a developer would need to specyfy a
Bundle with options, but currently they need to specify a lot of
additional values (all null) to do so. Having this API public makes it a
lot easier and less errorprone.
This change does not refactor existing uses where only the Bundle is
passed in.

Test: API change only
Bug: 273784196
Change-Id: I6486e953d21023cf5c8d120287205a3f9922bdd6