Update Robolectric SDK to 31

Update Robolectric SDK to 31

Bug: 206905117
Test: Manual
Change-Id: I918e3d29ecc7d853802a34bae0728fd737b9539b
diff --git a/robolectric_tests/config/robolectric.properties b/robolectric_tests/config/robolectric.properties
index 6a5d031..6c518c0 100644
--- a/robolectric_tests/config/robolectric.properties
+++ b/robolectric_tests/config/robolectric.properties
@@ -1,2 +1,2 @@
 manifest=packages/apps/ThemePicker/AndroidManifest.xml
-sdk=27
+sdk=31