bind the download engine by its interface in Koin

`single { SystemDownloadEngine(...) }` registers the concrete class, but
DownloadRepository asks for the DownloadEngine interface, so resolving
PlaybackController threw NoDefinitionFoundException and the app died on launch
with a Koin InstanceCreationException. It is now `single<DownloadEngine>`, the
way the other interface singletons in the module are declared.

Nothing in the suite ever started Koin — the repository tests build the
repository directly with a fake — which is why a launch-crashing binding got
through. AppModuleTest now resolves the playback graph from the real module, and
it failed with the same NoDefinitionFoundException before this fix.

app/src/test/kotlin/com/subtracks/di/AppModuleTest.kt

@@ -0,0 +1,33 @@
+package com.subtracks.di
+
+import android.content.Context
+import androidx.test.core.app.ApplicationProvider
+import androidx.test.ext.junit.runners.AndroidJUnit4
+import com.subtracks.data.repo.DownloadRepository
+import com.subtracks.playback.PlaybackController
+import okhttp3.OkHttpClient
+import org.junit.After
+import org.junit.Assert.assertNotNull
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.koin.core.context.startKoin
+import org.koin.core.context.stopKoin
+
+@RunWith(AndroidJUnit4::class)
+class AppModuleTest {
+    @Before
+    fun setUp() = stopKoin()
+
+    @After
+    fun tearDown() = stopKoin()
+
+    @Test
+    fun theModuleResolvesThePlaybackGraph() {
+        val context = ApplicationProvider.getApplicationContext<Context>()
+        val koin = startKoin { modules(appModule(context, OkHttpClient())) }.koin
+
+        assertNotNull(koin.get<DownloadRepository>())
+        assertNotNull(koin.get<PlaybackController>())
+    }
+}