[Android] Introducing mlc4j and app packaging

This PR lifts the existing `library` of android app into a standalone
`mlc4j` directory, which can be referenced by android app at any
location.

On the app side, this PR moves the android app into a subfolder
`MLCChat` which itself is a well-formed Android app. This folder
contains two core files for app build:

* `MLCChat/mlc-package-config.json` the config file that specifies
the models to build into the app.
* `MLCChat/prepare_package.py` the Python script that helps
automatically prepare/build mlc4j and model libraries.

This PR also updates the android app documentation to reflect this
latest change.

Author: MasterJH5574

android/MLCChat/README.md

@@ -0,0 +1,6 @@
+# MLC-LLM Android
+
+Checkout [Documentation page](https://llm.mlc.ai/docs/deploy/android.html) for more information.
+
+- run `python -m mlc_llm package`
+- open this `MLCChat/` folder as a project in Android Studio

android/app/.gitignore renamed to android/MLCChat/app/.gitignore

@@ -47,7 +47,7 @@ android {
 }
 
 dependencies {
-    implementation project(":library")
+    implementation project(":mlc4j")
     implementation 'androidx.core:core-ktx:1.10.1'
     implementation 'androidx.lifecycle:lifecycle-runtime-ktx:2.6.1'
     implementation 'androidx.activity:activity-compose:1.7.1'

android/app/build.gradle renamed to android/MLCChat/app/build.gradle

@@ -38,7 +38,7 @@ class AppViewModel(application: Application) : AndroidViewModel(application) {
     private val modelIdSet = emptySet<String>().toMutableSet()
 
     companion object {
-        const val AppConfigFilename = "app-config.json"
+        const val AppConfigFilename = "mlc-app-config.json"
         const val ModelConfigFilename = "mlc-chat-config.json"
         const val ParamsConfigFilename = "ndarray-cache.json"
         const val ModelUrlSuffix = "resolve/main/"