feat(template): add mobile projects, build script, fill initial src code (#305)

shared/template/android/src/main/java/ExamplePlugin.kt

@@ -0,0 +1,19 @@
+package {{android_package_id}}
+
+import android.app.Activity
+import app.tauri.annotation.Command
+import app.tauri.annotation.TauriPlugin
+import app.tauri.plugin.JSObject
+import app.tauri.plugin.Plugin
+import app.tauri.plugin.Invoke
+
+@TauriPlugin
+class ExamplePlugin(private val activity: Activity): Plugin(activity) {
+    @Command
+    fun ping(invoke: Invoke) {
+        val value = invoke.getString("value") ?: ""
+        val ret = JSObject()
+        ret.put("value", value)
+        invoke.resolve(ret)
+    }
+}