Fix AccessibilityButtonFragmentTest

**Root cause**
We expected the spy resources will be used when calling
context.getResources(). However, we didn't set up the connection. Hence
the test failed due to trying to use the original resources.

Bug: 279082331
Test: atest AccessibilityButtonFragmentTest
Change-Id: Ib6e51b2b2607bec7980b9288a0a8076e4a036863

tests/robotests/src/com/android/settings/accessibility/AccessibilityButtonFragmentTest.java

@@ -67,6 +67,7 @@ public class AccessibilityButtonFragmentTest {
     @Before
     public void setUp() {
         mFragment = spy(new TestAccessibilityButtonFragment(mContext));
+        when(mContext.getResources()).thenReturn(mResources);
         when(mFragment.getResources()).thenReturn(mResources);
         when(mFragment.getActivity()).thenReturn(Robolectric.setupActivity(FragmentActivity.class));
     }