Remove unnecessary ndk version indirection (#4755)

Summary:
[android] Remove unnecessary ndk version indirection
The constant was only referenced in one place, the ext property everywhere else.

Pull Request resolved: https://github.com/facebook/flipper/pull/4755

Test Plan:
CI

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* https://github.com/facebook/flipper/issues/4759
* https://github.com/facebook/flipper/issues/4758
* https://github.com/facebook/flipper/issues/4757
* https://github.com/facebook/flipper/issues/4756
* __->__ https://github.com/facebook/flipper/issues/4755
* https://github.com/facebook/flipper/issues/4754
* https://github.com/facebook/flipper/issues/4753
* https://github.com/facebook/flipper/issues/4752
* https://github.com/facebook/flipper/issues/4751

Reviewed By: ivanmisuno

Differential Revision: D46068954

Pulled By: passy

fbshipit-source-id: 276e4ee3d52a4a21c6dc29c17b6b2bd9968e945b

build.gradle

@@ -49,7 +49,7 @@ ext {
     targetSdkVersion = 31
     compileSdkVersion = 31
     buildToolsVersion = "30.0.3"
-    ndkVersion = "$NDK_VERSION"
+    ndkVersion = "25.1.8937393"
     javaTargetVersion = JavaVersion.VERSION_17
 }