Strings: add a username string.

app/src/main/res/layout/login_credentials_fragment.xml

@@ -37,7 +37,7 @@
                     android:id="@+id/user_name"
                     android:layout_width="match_parent"
                     android:layout_height="wrap_content"
-                    android:hint="@string/login_email_address"
+                    android:hint="@string/login_username"
                     android:autofillHints="emailAddress"
                     android:inputType="textEmailAddress"/>
             </com.google.android.material.textfield.TextInputLayout>

app/src/main/res/values/strings.xml

@@ -210,8 +210,9 @@
 
     <!-- AddAccountActivity -->
     <string name="login_title">Add account</string>
-    <string name="login_email_address">Username</string>
-    <string name="login_email_address_error">Valid username required</string>
+    <string name="login_username">Username</string>
+    <string name="login_email_address">Email</string>
+    <string name="login_email_address_error">Valid email required</string>
     <string name="login_password">Password</string>
     <string name="login_custom_server">EteSync Server URL</string>
     <string name="login_custom_server_error">Invalid URL found, did you forget to include https://?</string>