Fixes #8002
This is the second PR in the series that will be implementing the change username functionality.

Background

I made the UsernameChangerFullScreenDialogFragment abstract and added abstract methods so that the sign-up flow would have an implementation of it that retains the existing functionality while the new functionality for changing of the username in Account Settings would be able to create its own header behavior.

The main difference between the two components is as follows.

Sign-up Username Changer

• The header text should live update as the options are selected.
• The header text should contain both the username and display name.

Account Settings Username Changer

• The header text shouldn't update as the options are selected.
• The header text template will be similar to that of iOS.

Main Changes

• Abstract methods canHeaderTextLiveUpdate(), getSuggestionsFailedStat(), getHeaderText(username,display) were created.

• UsernameChangerFullScreenDialogFragment implemented the above methods by extending BaseUsernameChangerFullScreenDialogFragment

Testing

WPAndroid
Since the sign-up flow can't be reached in debug mode, the easiest way to test based on @theck13 suggestion is to modify the LoginActivity to go to the signup screen.
I did this modification at line 240

private void loggedInAndFinish(ArrayList<Integer> oldSitesIds, boolean doLoginUpdate) {
    ActivityLauncher.showMainActivityAndSignupEpilogue(this, "John", "John Doe", "fake_url","johndoe876");
  ... rest of function commented out.

There's probably an easier way to do it, but I was hoping that doing it after login would give me a token so I could make changes.

A gif of the behaviour when tested :

UI Tests
I wrote an Espresso test that created an environment where the fragment could be spun up in isolation in an activity container with a mocked instance of the dispatcher. The test basically verifies that the functionality that has been modified is still intact and it will be instrumental in verifying the behavior of the followup settings username changer fragment.

Main Changes
To accomplish this these were the steps:

• The fragment injector on the application class was given a setter so that it could be replaced with a test variant.
• A FragmentActivity element was added to the debug manifest so that a registered activity would be available as a container.
• @ContributesAndroidInjector and DaggerFragment was utilized for the fragment DI so the injector could be replaced easily and the mocked dispatcher could be injected.

Update release notes:

• If there are user-facing changes, I have added an item to RELEASE-NOTES.txt.