Remove dependency on platform-system-robolectric

Dependency on platform-system-robolectric is not needed when mockito
is not necessary.
If mockito is needed, add a static dependency on
mockito-robolectric-prebuilt.

Test: cd tests/robotests && mma
Bug: 38456058
Change-Id: I6fa83e1f4cea55845dbb3369eda4065d93e85d35
diff --git a/tests/robotests/Android.mk b/tests/robotests/Android.mk
index 2ebc12f..6ec2e93 100644
--- a/tests/robotests/Android.mk
+++ b/tests/robotests/Android.mk
@@ -8,7 +8,6 @@
 
 # Include the testing libraries (JUnit4 + Robolectric libs).
 LOCAL_STATIC_JAVA_LIBRARIES := \
-    platform-system-robolectric \
     truth-prebuilt
 
 LOCAL_JAVA_LIBRARIES := \