use MEDIA_* events to update USB-OTG detection

This means that sometimes the NearbyView is updated from a
BroadcastReceiver's Context, which is not an Activity.  So this has to
try a little harder to fetch the Activity instance needed for the
prompt to request permissions to a folder on the USB.  This adds a
failsafe to fallback to the file:/// scanning in SDCardScannerService.

app/src/full/AndroidManifest.xml

@@ -100,7 +100,6 @@
                     android:name="android.hardware.usb.action.USB_DEVICE_ATTACHED"
                     android:resource="@xml/device_filter"/>
         </receiver>
-
         <receiver
                 android:name=".nearby.UsbDeviceDetachedReceiver">
             <intent-filter>
@@ -110,6 +109,17 @@
                     android:name="android.hardware.usb.action.USB_DEVICE_DETACHED"
                     android:resource="@xml/device_filter"/>
         </receiver>
+        <receiver android:name=".nearby.UsbDeviceMediaMountedReceiver">
+            <intent-filter>
+                <action android:name="android.intent.action.MEDIA_EJECT" />
+                <action android:name="android.intent.action.MEDIA_REMOVED" />
+                <action android:name="android.intent.action.MEDIA_MOUNTED" />
+                <action android:name="android.intent.action.MEDIA_BAD_REMOVAL" />
+
+                <data android:scheme="content" />
+                <data android:scheme="file" />
+            </intent-filter>
+        </receiver>
 
         <activity
                 android:name=".panic.PanicPreferencesActivity"

app/src/full/java/org/fdroid/fdroid/nearby/UsbDeviceMediaMountedReceiver.java

@@ -0,0 +1,23 @@
+package org.fdroid.fdroid.nearby;
+
+import android.content.BroadcastReceiver;
+import android.content.Context;
+import android.content.Intent;
+import android.os.Environment;
+import org.fdroid.fdroid.views.main.NearbyViewBinder;
+
+public class UsbDeviceMediaMountedReceiver extends BroadcastReceiver {
+    @Override
+    public void onReceive(Context context, Intent intent) {
+        if (intent == null || intent.getAction() == null) {
+            return;
+        }
+        String action = intent.getAction();
+        if (Environment.MEDIA_BAD_REMOVAL.equals(action)
+                || Environment.MEDIA_MOUNTED.equals(action)
+                || Environment.MEDIA_REMOVED.equals(action)
+                || Environment.MEDIA_EJECTING.equals(action)) {
+            NearbyViewBinder.updateUsbOtg(context);
+        }
+    }
+}

app/src/full/java/org/fdroid/fdroid/views/main/NearbyViewBinder.java

@@ -196,8 +196,23 @@ public class NearbyViewBinder {
                                 return;
                             }
                         }
-                        ((Activity) context).startActivityForResult(intent,
+
-                            MainActivity.REQUEST_STORAGE_ACCESS);
+                        Activity activity = null;
+                        if (context instanceof Activity) {
+                            activity = (Activity) context;
+                        } else if (swapView != null && swapView.getContext() instanceof Activity) {
+                            activity = (Activity) swapView.getContext();
+                        }
+
+                        if (activity != null) {
+                            activity.startActivityForResult(intent, MainActivity.REQUEST_STORAGE_ACCESS);
+                        } else {
+                            // scan in the background without requesting permissions
+                            Toast.makeText(context.getApplicationContext(),
+                                    context.getString(R.string.scan_removable_storage_toast, externalStorage),
+                                    Toast.LENGTH_SHORT).show();
+                            SDCardScannerService.scan(context);
+                        }
                     }
                 });
             }