Get Robolectric tests passing

+ Default SDK 28 -> 26, as >26 isn't available for
  unbundled make builds.

+ @Ignore 3 test methods that currently fail with
  resource-related errors.

Test: RunLauncherRoboTests succeeds locally
Bug: 119567714
Change-Id: Id6ea42cef050119d892b5af2b1ba1afc4c4f0b28

robolectric_tests/src/com/android/launcher3/util/IntSetTest.java

@@ -21,6 +21,7 @@ import org.junit.Test;
 import org.junit.runner.RunWith;
 
 import org.robolectric.RobolectricTestRunner;
+import org.robolectric.annotation.Config;
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;