Add an explicit NDK version to Android template (#29403)

Summary:
Added an explicit NDK version to the Android template. This allows tooling to detect which NDK version to install automatically.

## Changelog

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[Android] [Added] - Add an explicit NDK version to Android template

Pull Request resolved: https://github.com/facebook/react-native/pull/29403

Test Plan: Template builds as it should. NDK version gets installed with initial Android set up.

Reviewed By: passy

Differential Revision: D23752007

Pulled By: fkgozali

fbshipit-source-id: 31c33f275f94a4a62338a61e79b31c4b996969bf

template/android/app/build.gradle

@@ -121,6 +121,8 @@ def jscFlavor = 'org.webkit:android-jsc:+'
 def enableHermes = project.ext.react.get("enableHermes", false);
 
 android {
+    ndkVersion rootProject.ext.ndkVersion
+
     compileSdkVersion rootProject.ext.compileSdkVersion
 
     compileOptions {

template/android/build.gradle

@@ -6,6 +6,7 @@ buildscript {
         minSdkVersion = 19
         compileSdkVersion = 29
         targetSdkVersion = 29
+        ndkVersion = "20.1.5948944"
     }
     repositories {
         google()