Annotate RemoteInput#SOURCE_* with @IntDef

This patch addresses a request from the API Council (http://b/74409378).
It defines a new RemoteInput.Source annotation for RemoteInput source
constants.

Bug: 74409378
Test: atest RemoteInputTest
Change-Id: I78a006b6a600ea0c0603b2591d1a29596074a44e
1 file changed