Update Robolectric to stable version 4.3.

We currently use an alpha version which allowed us to access new threading
features. The stable version of this has been released now and we can
switch back.

PiperOrigin-RevId: 257149681

constants.gradle

@@ -20,7 +20,7 @@ project.ext {
     compileSdkVersion = 28
     dexmakerVersion = '2.21.0'
     mockitoVersion = '2.25.0'
-    robolectricVersion = '4.3-alpha-2'
+    robolectricVersion = '4.3'
     autoValueVersion = '1.6'
     autoServiceVersion = '1.0-rc4'
     checkerframeworkVersion = '2.5.0'