Make getCurrentActivity public on ReactContext

Summary:
Addresses #8661
Closes https://github.com/facebook/react-native/pull/9071

Differential Revision: D3641285

Pulled By: foghina

fbshipit-source-id: dede86743efddc33b6ead053e805770fc213685c

ReactAndroid/src/main/java/com/facebook/react/bridge/ReactContext.java

@@ -283,7 +283,7 @@ public class ReactContext extends ContextWrapper {
    * DO NOT HOLD LONG-LIVED REFERENCES TO THE OBJECT RETURNED BY THIS METHOD, AS THIS WILL CAUSE
    * MEMORY LEAKS.
    */
-  /* package */ @Nullable Activity getCurrentActivity() {
+  public @Nullable Activity getCurrentActivity() {
     if (mCurrentActivity == null) {
       return null;
     }