[android] keep strong callback reference and use application context during cache path change

platform/android/MapboxGLAndroidSDKTestApp/src/androidTest/java/com/mapbox/mapboxsdk/testapp/storage/FileSourceMapTest.kt

@@ -36,12 +36,12 @@ open class FileSourceMapTest {
   fun changeResourcesPathWhileMapVisible() {
     val latch = CountDownLatch(1)
     rule.activity.runOnUiThread {
-      FileSource.setResourcesCachePath(rule.activity, fileSourceTestUtils.testPath, object : FileSource.ResourcesCachePathChangeCallback {
-        override fun onSuccess(path: String?) {
+      FileSource.setResourcesCachePath(fileSourceTestUtils.testPath, object : FileSource.ResourcesCachePathChangeCallback {
+        override fun onSuccess(path: String) {
           Assert.fail("Requested resources change while the map is running should fail")
         }
 
-        override fun onError(message: String?) {
+        override fun onError(message: String) {
           Assert.assertEquals("Cannot set path, file source is activated."
             + " Make sure that the map or a resources download is not running.", message)
           latch.countDown()

platform/android/MapboxGLAndroidSDKTestApp/src/androidTest/java/com/mapbox/mapboxsdk/testapp/storage/FileSourceTestUtils.kt

@@ -33,14 +33,13 @@ class FileSourceTestUtils(private val activity: Activity) {
     val latch = CountDownLatch(1)
     activity.runOnUiThread {
       FileSource.setResourcesCachePath(
-        activity,
         path,
         object : FileSource.ResourcesCachePathChangeCallback {
-          override fun onSuccess(path: String?) {
+          override fun onSuccess(path: String) {
             latch.countDown()
           }
 
-          override fun onError(message: String?) {
+          override fun onError(message: String) {
             Assert.fail("Resource path change failed - path: $path, message: $message")
           }
         })

platform/android/MapboxGLAndroidSDKTestApp/src/main/java/com/mapbox/mapboxsdk/testapp/activity/offline/ChangeResourcesCachePathActivity.kt

@@ -32,6 +32,8 @@ class ChangeResourcesCachePathActivity : AppCompatActivity(),
 
   private lateinit var offlineManager: OfflineManager
 
+  private val callback = PathChangeCallback(this)
+
   override fun onCreate(savedInstanceState: Bundle?) {
     super.onCreate(savedInstanceState)
     setContentView(R.layout.activity_change_resources_cache_path)
@@ -50,18 +52,23 @@ class ChangeResourcesCachePathActivity : AppCompatActivity(),
     Toast.makeText(this, "Current path: $path", Toast.LENGTH_LONG).show()
   }
 
+  override fun onDestroy() {
+    super.onDestroy()
+    callback.onActivityDestroy()
+  }
+
   override fun onItemClick(parent: AdapterView<*>?, view: View?, position: Int, id: Long) {
     listView.onItemClickListener = null
     val path: String = adapter.getItem(position) as String
-    FileSource.setResourcesCachePath(this, path, this)
+    FileSource.setResourcesCachePath(path, callback)
   }
 
-  override fun onError(message: String?) {
+  override fun onError(message: String) {
     listView.onItemClickListener = this
     Toast.makeText(this, "Error: $message", Toast.LENGTH_LONG).show()
   }
 
-  override fun onSuccess(path: String?) {
+  override fun onSuccess(path: String) {
     listView.onItemClickListener = this
     Toast.makeText(this, "New path: $path", Toast.LENGTH_LONG).show()
 
@@ -118,6 +125,21 @@ class ChangeResourcesCachePathActivity : AppCompatActivity(),
     return paths
   }
 
+  private class PathChangeCallback(private var activity: ChangeResourcesCachePathActivity?) : FileSource.ResourcesCachePathChangeCallback {
+
+    override fun onSuccess(path: String) {
+      activity?.onSuccess(path)
+    }
+
+    override fun onError(message: String) {
+      activity?.onError(message)
+    }
+
+    fun onActivityDestroy() {
+      activity = null
+    }
+  }
+
   class PathAdapter(private val context: Context, private val paths: List<String>) : BaseAdapter() {
 
     override fun getItem(position: Int): Any {