Make AndroidCamera compatible with android API >= 24

[antocuni]

This should finally fix #500 .

On modern android versions, the only way use the ACTION_IMAGE_CAPTURE intent is to use fileproviders, and somehow we need to tell plyer which fileprovider authority to use. I don't think there is a general mechanism to configure per-platform settings in plyer, so in the PR I adopted for the following API:

from kivy.utils import platform
import plyer
if platform == 'android':
    plyer.camera.FILEPROVIDER_AUTHORITY = '...'
plyer.camera.take_picture(filename, ...)

This works as long as filename has a proper configured fileprovider, which is not possible with the current p4a version.
The easiest way is to use kivy/python-for-android#1922.

I also published a fully working example which combines this PR, kivy/python-for-android#1922 and kivy/buildozer#1369:
https://github.com/antocuni/plyer_camera_example