Add "results source" parameter to RemoteInput

This CL adds the following methods to RemoteInput:

  * setResultsSource(Intent intent, int source)
  * getResultsSource(Intent intent)

where source can be one of:

  * RemoteInput.SOURCE_FREE_FORM_INPUT
  * RemoteInput.SOURCE_CHOICE

Rationale: This will allow apps to distinguish direct and smart replies
for logging purposes.

Bug: 67765414
Test: atest RemoteInputTest (http://ag/3469835)
Change-Id: Ie30416640df4b2cd0424f9c75b235fc124be87a3