Preference to disable multiple-login usernames (#2718)

* Preference to disable multiple-login usernames (with problems)

* Fix problem where 'show self username disambiguation' does not take effect immediately because MainActivity needed to be restarted

* Make 'show username in toolbars' a 3-option selector, default when multiple accounts logged in

* Move SHOW_SELF_USERNAME higher in preference fragment

app/src/main/java/com/keylesspalace/tusky/components/compose/ComposeActivity.kt

@@ -244,7 +244,7 @@ class ComposeActivity :
 
         viewModel.setup(composeOptions)
 
-        if (accountManager.accounts.size > 1) {
+        if (accountManager.shouldDisplaySelfUsername(this)) {
             binding.composeUsernameView.text = getString(
                 R.string.compose_active_account_description,
                 activeAccount.fullName