Add @FlakyTest annotation to @Postsubmit tests

@Postsubmit may be used for something else in the future, so replace the
annotation with the more correct and specific annotation @FlakyTest

Bug: 35710737
Test: No change to behavior
Change-Id: I740c2c9e3cfcd78481d4a6f8f179872cd7832beb
diff --git a/tests/Android.mk b/tests/Android.mk
index f4b8256..1e8b1af 100644
--- a/tests/Android.mk
+++ b/tests/Android.mk
@@ -20,6 +20,7 @@
 LOCAL_STATIC_JAVA_LIBRARIES := \
         android-ex-camera2 \
         android-support-v4 \
+        android-support-test \
         guava \
         mockito-target \
         platform-test-annotations \