Fix DataSaverBackend objects created on background threads

Add the main looper to the handler created with each
DataSaverBackend to avoid crashes when the objects are
created on background threads.

Bug: 62022517
Test: make RunSettingsRoboTests
Change-Id: I7396107e4ed06982c8cd300912ce1f4e3c63df4c
Merged-In: Ie5ffabbfbe7660761527b3ecd51e6bc5a43c1ace

tests/robotests/src/com/android/settings/testutils/shadow/ShadowDataSaverBackend.java

@@ -0,0 +1,21 @@
+package com.android.settings.testutils.shadow;
+
+import com.android.settings.datausage.DataSaverBackend;
+import org.robolectric.annotation.Implementation;
+import org.robolectric.annotation.Implements;
+
+@Implements(DataSaverBackend.class)
+public class ShadowDataSaverBackend {
+
+    private static boolean isEnabled = true;
+
+    @Implementation
+    public boolean isDataSaverEnabled() {
+        return isEnabled;
+    }
+
+    @Implementation
+    public void setDataSaverEnabled(boolean enabled) {
+        isEnabled = enabled;
+    }
+}