android: use file to override native modules build

Reads the contents of a react-native project's file for overriding
the automatic native modules detection.

android/build.gradle

@@ -106,6 +106,14 @@ import org.gradle.internal.os.OperatingSystem;
 
 String shouldRebuildNativeModules = System.getenv('NODEJS_MOBILE_BUILD_NATIVE_MODULES');
 
+if (shouldRebuildNativeModules==null) {
+// If the environment variable is not set right now, check if it has been saved to a file.
+    def nativeModulesPreferenceFile = file("${rootProject.projectDir}/../nodejs-assets/BUILD_NATIVE_MODULES.txt");
+    if (nativeModulesPreferenceFile.exists()) {
+        shouldRebuildNativeModules=nativeModulesPreferenceFile.text.trim();
+    }
+}
+
 if (shouldRebuildNativeModules==null) {
 // If build native modules preference is not set, try to find .gyp files to turn it on.
     shouldRebuildNativeModules="0";