Add user-defined button labels with color change on press

- PrefsHelper: PREF_LABEL1, PREF_LABEL2 constants
- SettingsActivity: EditText fields with Done-keyboard action to save labels
- MainActivity: buttons show user label, colorAccent on press, textAllCaps disabled
- activity_main.xml: textAllCaps=false for case-sensitive labels

android/app/src/main/res/layout/activity_main.xml

@@ -26,6 +26,7 @@
             android:layout_height="200dp"
             android:text="1"
             android:textSize="24sp"
+            android:textAllCaps="false"
             android:enabled="false" />
 
         <Button
@@ -35,6 +36,7 @@
             android:layout_marginStart="16dp"
             android:text="2"
             android:textSize="24sp"
+            android:textAllCaps="false"
             android:enabled="false" />
 
     </LinearLayout>