Use initial value on save when value didn't change.

Otherwise, it will fail to call onSaveRequest() in scenarios where the service
required a field that was pre-populated for the app. For example, when the
service requires both a username and password, the app pre-populates the
username, and the user fills the password.

Test: new LoginActivityTest.testSaveOnlyTwoRequiredFieldsOnePrefilled() pass
Test: existing CtsAutoFillServiceTestCases pass

Bug: 62270740

Change-Id: Idbb11a8c2dab6741c18549c55dfdb71b483c2447