Fix race condition in ReadableNativeMap

Summary:
[Changelog][Internal]

Guard call to the C++ ReadableNAtiveMap.importValues with a lock.

Note that all the occurrences in this class (together with importTypes) already were protected by a lock, except of this one, which with the very high chance caused crashes in T145271136.

My corresponding comment from the task,  for justification:
> If callstack to be trusted, the crash happens on the C++ side, in ReadableNativeMap::importValues().
It throws ArrayIndexOutOfBoundsException, which, looking at the code, seems to be only possible due to a corrupted data or race conditions.

> Now, looking at the Java side of ReadableNativeMap, and the particular call site... it's very dodgy, since all other occurrences of calling to native importTypes/importValues are guarded by locks, but the one crashing isn't.

NOTE: A couple of `importKeys()` instances appears to suffer from the same problem as well.

Reviewed By: javache

Differential Revision: D43398416

fbshipit-source-id: 5ea3301857517d99a9007ddf2739e49ca8136743

Author: rshest

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

@@ -188,10 +188,15 @@ public int getInt(@NonNull String name) {
   @Override
   public @NonNull Iterator<Map.Entry<String, Object>> getEntryIterator() {
     if (mKeys == null) {
-      mKeys = Assertions.assertNotNull(importKeys());
+      synchronized (this) {
+        mKeys = Assertions.assertNotNull(importKeys());
+      }
     }
     final String[] iteratorKeys = mKeys;
-    final Object[] iteratorValues = Assertions.assertNotNull(importValues());
+    Object[] iteratorValues;
+    synchronized (this) {
+      iteratorValues = Assertions.assertNotNull(importValues());
+    }
     return new Iterator<Map.Entry<String, Object>>() {
       int currentIndex = 0;
 
@@ -227,7 +232,9 @@ public Object setValue(Object value) {
   @Override
   public @NonNull ReadableMapKeySetIterator keySetIterator() {
     if (mKeys == null) {
-      mKeys = Assertions.assertNotNull(importKeys());
+      synchronized (this) {
+        mKeys = Assertions.assertNotNull(importKeys());
+      }
     }
     final String[] iteratorKeys = mKeys;
     return new ReadableMapKeySetIterator() {