NDK version from gradle.properties

Summary:
^

With these changes, we can build the project by overriding these version from a gradle build.

    ./gradlew :assembleDebug -DNDK_VERSION=21.4.7075529 -DFBJNI_VERSION=0.2.2

Reviewed By: passy

Differential Revision: D43879098

fbshipit-source-id: 8790aa12a086373194d45687d70ceb73b07713b9

react-native/ReactNativeFlipperExample/android/build.gradle

@@ -12,7 +12,7 @@ buildscript {
         minSdkVersion = 21
         compileSdkVersion = 31
         targetSdkVersion = 31
-        ndkVersion = '23.0.7599858'
+        ndkVersion = "$NDK_VERSION"
     }
     repositories {
         google()