Use react.gradle from repo root instead of copy in RNTester

Summary:
RNTester used a copy of the main react.gradle file instead of the real one. Recent changes to the real file caused builds to fail with older gradle versions but was not caught by CI because it wasn't using that file for RNTester. That copy of react.gradle is just a leftover from when projects included a copy instead of importing the one in RN directly.

Note: CI WILL fail with this PR, if we have trouble landing this I can add the revert in this commit too but wanted to keep it as 2 separate commits.

Tested that building RNTester actually fails now that it uses react.gradle with recent changes, then tested that is builds properly when reverting d16ff3bd8b92fa84a9007bf5ebedd8153e4c089d.

[INTERNAL] [MINOR] [RNTester] - Use react.gradle from repo root instead of copy in RNTester
Closes facebook/react-native#18188

Differential Revision: D7155179

Pulled By: hramos

fbshipit-source-id: 15b461a63b841bf807e7d11ba3ead005ca5e33b0

Author: janicduplessis

android/app/build.gradle

@@ -64,7 +64,7 @@ project.ext.react = [
     inputExcludes: ["android/**", "./**"]
 ]
 
-apply from: "react.gradle"
+apply from: "../../../react.gradle"
 
 /**
  * Set this to true to create three separate APKs instead of one: