enable CI for pull request (facebook#31)

Summary:
## Motivation

Why are you making this change?

What did you change?

How does the code work?

Why did you choose this approach?
Pull Request resolved: facebookincubator/fbjni#31

Test Plan:
How did you test this change?
Any change that adds functionality should add a unit test as well.

Reviewed By: BurntBrunch

Differential Revision: D19348810

Pulled By: passy

fbshipit-source-id: 7a144c80cc1078d968a562a17fa8d5f84047eeb2

first-party/fbjni/.github/workflows/android.yml

@@ -1,6 +1,6 @@
 name: Android CI
 
-on: [push]
+on: [push, pull_request]
 
 jobs:
   build: