Fix: Use "rwt" file mode for the output image file

[AdamGrzybkowski]

Closes #546

This PR fixes an issue with the output file size. No matter how low the compression quality is set the image will remain its original file size after cropping.

The default file mode used when opening the OutputStream for the mImageOutputUri seems to be the culprit. Changing it to wt fixed the issue.

Testing:

Use the sample app and first check the develop-non-native branch.

1. Use Tmp file destination
2. Select Resize image to max size - set it to a low value like 500
3. Set compression quality to 20
4. Tap pick & crop
5. Use JPEG
6. Select an image (big one ideally)
7. Crop the image
8. Once finished use the Device Explorer in Android Studio and go to data/data/com.yalantis.ucrop.sample/cache
9. Check the size of the file SampleCropImage.jpg - compare it with the original

Now check out this branch and repeat the steps above.

[AdamGrzybkowski]

@lynnik It looks like you are one of the maintainers now - can you take a look at this one?

[maxme]

Hey @lynnik, this is kind of blocking for us and we'll probably have to fork and manage our own release of uCrop if we want to continue working with it. Obviously this is not something we would like to do.

Let us know (@AdamGrzybkowski , @hamorillo and @maxme) if you need help with the project and publishing updates, maybe we can help.

[vladlynnik]

Hey, @AdamGrzybkowski, thanks for reaching out! We’re currently working on it and will provide an update soon. We'll keep you in the loop.

[jens-muenker]

I made today a non-native version of my fork and included your bugfix. Maybe that might help you :)