Fix gitignore

build/ is too generic, ignores valid paths.

Test plan:
- Built Android UI explorer, ReactAndroid, iOS UIExplorer, iOS React
- Git still ignores build output files correctly

.gitignore

@@ -21,8 +21,10 @@ DerivedData
 *.xcuserstate
 project.xcworkspace
 
-# Xcode, Gradle
-build/
+# Gradle
+/build/
+/Examples/**/android/app/build/
+/ReactAndroid/build
 
 # Android
 .idea

ReactAndroid/src/main/java/com/facebook/react/common/build/ReactBuildConfig.java

@@ -0,0 +1,24 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+package com.facebook.react.common.build;
+
+import com.facebook.react.BuildConfig;
+
+/**
+ * Convenience class for accessing auto-generated BuildConfig so that a) other modules can just
+ * depend on this module instead of having to manually depend on generating their own build config
+ * and b) we don't have to deal with IntelliJ getting confused about the autogenerated BuildConfig
+ * class all over the place.
+ */
+public class ReactBuildConfig {
+
+  public static final boolean DEBUG = BuildConfig.DEBUG;
+  public static final boolean IS_INTERNAL_BUILD = BuildConfig.IS_INTERNAL_BUILD;
+}