media: Mark public MediaHandler/ImageHandler methods as such

media: Mark public MediaHandler/ImageHandler methods as such

Note I'm intentionally not touching the entire file, but only methods
I'm absolutely sure are already called from outside, e.g. from
MediaHandlerFactory, and must be public because of this.

I'm intentionally not doing anything with private or protected in this
patch, as such changes are much more fragile.

This is a direct follow up for the changes proposed in Iaa4f60d.

Change-Id: Ida817b289ddd5e9a8c162cc1fa3335c639a0bbe5