Minor cleanups and fixes for ActionIntentCreator

Makes parameters for text and subject non-nullable
Don't add null values to the output intent
Shorten function names by removing 'Intent' suffix
Tests use IntentSubject (Truth) where possible

Bug: 291706486
Fixes: 294241612
Test: atest ActionIntentCreatorTest
Change-Id: I39a4f70d61a8c57c01d8cf0e98ad64e66f7be217