Document how to determine if an app is a flutter app (#12668)

_Description of what this PR is changing or adding, and why:_

Documents how to tell from a built apk if the app uses flutter. 
Internal documentation authored by me can be seen by googlers in
[go/flutter-android-detection](http://goto.google.com/flutter-android-detection)

_Issues fixed by this PR (if any):_
flutter/flutter/issues/170523

_PRs or commits this PR depends on (if any):_
n/a

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---------

Co-authored-by: Shams Zakhour <[email protected]>

Author: reidbaker

src/content/deployment/android.md

@@ -636,6 +636,50 @@ panel (debug is the default):
 The resulting app bundle or APK files are located in
 `build/app/outputs` within your app's folder.
 
+### How to tell if an apk uses Flutter?
+
+Recommended: Using APK files
+[apkanalyzer](https://developer.android.com/tools/apkanalyzer) files list --files-only <SOME-APK>
+Then looking for a file in `/lib/<ARCH>/libflutter.so`
+
+Example:
+`apkanalyzer files list some-flutter-app.apk | grep flutter.so | wc -l`
+returns any number greater than 0.
+
+**Why this works**
+Flutter depends on C++ code used by the Flutter engine. In Android,
+this code is bundled with the Flutter framework and the developer's
+Dart code as a native library called `libflutter.so`.
+The Java/Android tooling renames the `flutter` library with the `lib` prefix
+and handles library location across architectures.
+This is how some reverse engineer an APK to identify it as a Flutter app.
+
+#### Secondary Evaluation:
+Run `apkanalyzer manifest print <SOME-APK>` and look for a `<meta-data>` tag with `android:name="flutterEmbedding"`.
+The value can be `1` or `2`.
+
+Example:
+`apkanalyzer manifest print some-flutter-app.apk | grep flutterEmbedding -C 2`
+returns the following style string.
+```
+<meta-data
+   android:name="flutterEmbedding"
+   android:value="2" />
+```
+
+
+**Why this works**
+Flutter has had two different embedders, and this flag was read to determine which embedder was used.
+Flutter 3.22 removed the ability of v1 embedder apps to build.
+https://blog.flutter.dev/whats-new-in-flutter-3-22-fbde6c164fe3
+This mechanism is not recommended because it is unclear how long the `flutterEmbedding` value will
+continue to be included in all Flutter apps. Additionally, this will not work for all libraries written
+in Flutter that are imported into Android apps as AAR dependencies.
+
+#### Non-technical evaluation
+*   Download [Flutter Shark](https://play.google.com/store/apps/details?id=com.fluttershark.fluttersharkapp&pli=1) on a device and let it scan local apps.
+*   Visit the [Flutter Hunt](https://flutterhunt.com/) website.
+
 {% comment %}
 ### Are there any special considerations with add-to-app?
 {% endcomment %}