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

shared/template/android/src/androidTest/java/ExampleInstrumentedTest.kt

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+package {{android_package_id}}
+
+import androidx.test.platform.app.InstrumentationRegistry
+import androidx.test.ext.junit.runners.AndroidJUnit4
+
+import org.junit.Test
+import org.junit.runner.RunWith
+
+import org.junit.Assert.*
+
+/**
+ * Instrumented test, which will execute on an Android device.
+ *
+ * See [testing documentation](http://d.android.com/tools/testing).
+ */
+@RunWith(AndroidJUnit4::class)
+class ExampleInstrumentedTest {
+    @Test
+    fun useAppContext() {
+        // Context of the app under test.
+        val appContext = InstrumentationRegistry.getInstrumentation().targetContext
+        assertEquals("{{android_package_id}}", appContext.packageName)
+    }
+}

shared/template/android/src/main/AndroidManifest.xml

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+<?xml version="1.0" encoding="utf-8"?>
+<manifest xmlns:android="http://schemas.android.com/apk/res/android">
+</manifest>

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

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+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)
+    }
+}

shared/template/android/src/test/java/ExampleUnitTest.kt

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+package {{android_package_id}}
+
+import org.junit.Test
+
+import org.junit.Assert.*
+
+/**
+ * Example local unit test, which will execute on the development machine (host).
+ *
+ * See [testing documentation](http://d.android.com/tools/testing).
+ */
+class ExampleUnitTest {
+    @Test
+    fun addition_isCorrect() {
+        assertEquals(4, 2 + 2)
+    }
+}