Update all preferences when the account preference updates UI

When an account is added or removed, the account preference controller
just updates the account list, whereas there's no trigger point for the
account data sync preferences to refresh.

Add an API to update all preferences in DashboardFragment, so we can
refresh the account sync preferences whenever the account list is
updated.

Bug: 185479221
Test: robotest, manual
Change-Id: Ide3d972abae09ce0175e3bafbb1f4a5e307d20c8

tests/robotests/src/com/android/settings/dashboard/DashboardFragmentTest.java

@@ -297,6 +297,14 @@ public class DashboardFragmentTest {
         mTestFragment.updatePreferenceVisibility(prefControllers);
     }
 
+    @Test
+    public void forceUpdatePreferences_prefKeyNull_shouldNotCrash() {
+        mTestFragment.addPreferenceController(new TestPreferenceController(mContext));
+
+        // Should not crash
+        mTestFragment.forceUpdatePreferences();
+    }
+
     @Test
     public void checkUiBlocker_noUiBlocker_controllerIsNull() {
         mTestFragment.mBlockerController = null;