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

build.gradle

@@ -48,8 +48,8 @@ ext {
     minSdkVersion = 15
     targetSdkVersion = 31
     compileSdkVersion = 31
-    buildToolsVersion = '30.0.3'
-    ndkVersion = '23.0.7599858'
+    buildToolsVersion = "30.0.3"
+    ndkVersion = "$NDK_VERSION"
 }
 
 ext.deps = [