Handle WRITE_EXTERNAL_STORAGE Android permission

While porting QtMultimedia to new QPermission API, I noticed that the Android backend uses WRITE_EXTERNAL_STORAGE permission. This permission does not have an exact equivalent on other platforms, so it does not look like a good candidate for extending QPermission.

But we could think of some platform-specific public API to query this permission. Then the library code can be updated to check for this permission.
If that is done, at least the AudioRecorder and AudioPanning examples should be updated to request this permission.