android/permission: Add prominent disclosure for location permission

Google Play Store have a requirement for permission android.permission.ACCESS_BACKGROUND_LOCATION which needs to in-app disclosure to explain why we need it
https://support.google.com/googleplay/android-developer/answer/9799150

NEWS.txt

@@ -3,6 +3,8 @@ Version 7.21 - not yet released
   - Support for AF/AR frequency fields in OpenAir
 * devices
   - fix inverted cruise/circling mode in borgelt and xcvario driver
+* Android
+  - re-enable background location and comply with Google Play Store policy
 
 Version 7.20 - 2021/10/22
 * map

android/src/XCSoar.java

@@ -24,7 +24,9 @@ of the License, or (at your option) any later version.
 
 import android.Manifest;
 import android.app.Activity;
+import android.app.AlertDialog;
 import android.app.PendingIntent;
+import android.content.DialogInterface;
 import android.os.Bundle;
 import android.view.MotionEvent;
 import android.view.KeyEvent;
@@ -270,11 +272,20 @@ private void requestAllPermissions() {
        is not enough */
 
     if (!hasAllPermissions()) {
-      try {
-        this.requestPermissions(NEEDED_PERMISSIONS, 0);
-      } catch (IllegalArgumentException e) {
-        Log.e(TAG, "could not request permissions: " + String.join(", ", NEEDED_PERMISSIONS), e);
-      }
+      new AlertDialog.Builder(this)
+        .setTitle("Location Permission")
+        .setMessage("XCSoar needs to collect location data to enable live navigation calculation and IGC logger, even when the app is in the background")
+        .setPositiveButton("Continue", new DialogInterface.OnClickListener() {
+            @Override
+            public void onClick(DialogInterface dialog, int which) {
+              try {
+                XCSoar.this.requestPermissions(NEEDED_PERMISSIONS, 0);
+              } catch (IllegalArgumentException e) {
+                Log.e(TAG, "could not request permissions: " + String.join(", ", NEEDED_PERMISSIONS), e);
+              }
+            }
+        })
+        .show();
     }
   }