Integrate or create an image picker to allow multiple image selection and upload

[bummytime]

Original issue: https://github.com/wordpress-mobile/WordPress-Android/issues/1628

We can't import multiple images from the device to the app (the only way is to use the "share via WordPress" feature). We should create an image picker/browser that allow multiple image import and make it available from media manager and new post views.

Rethink the app flow to allow gallery creation after multiple image import.

[maxme]

We now have a media picker, but it needs some work to replace the old picker completely. IMO, we should drop the media picker and use the ACTION_PICK intent with EXTRA_ALLOW_MULTIPLE that will invoke the native Android picker - only works on API level > 18 (sorry for other users).