android : update CMakeLists.txt to use FetchContent for ggml (#3268)

android : update CMakeLists.txt to use FetchContent for ggml (#3268)

* android : update CMakeLists.txt to use FetchContent for ggml

This commit updates the CMakeLists.txt file for the Android Whisper
example to use FetchContent for managing the ggml library.

The motivation for this change is avoid having to make manual changes to
the CMakeLists.txt file after syncing the ggml library.

I've built and run the example locally to verify that it works as
expected.

Refs: https://github.com/ggml-org/whisper.cpp/pull/3265#issuecomment-2986715717

* android.java : update cmake to use FetchContent for ggml

This commit updates the CMake configuration for the Android Java example
to use `FetchContent` for including the `ggml` library. Do be able to
use FetchContent we also update the `compileSdkVersion` and
`targetSdkVersion` to 31, and the `buildToolsVersion` to '30.0.3'.
This also required a an update to the Gradle plugin version to 7.4.0.

The motivation for this change is avoid having to make manual changes to
the CMakeLists.txt file after syncing the ggml library.