Updates camera.md

Following ag/19119857, this existing piece of documentation is updated.
Note that most of the class names and links in the document were
incorrect, probably due to drift between refactors that renamed stuff
(like `StatusBar.java`) and this document.

Where it was possible and sensical, I updated the document. Where it
wasn't, I removed specifics related to class/method names or line
numbers. The general gist of how the double-click on the power button
ends up launching the camera is still correct.

Bug: b/235403546
Test: N/A
Change-Id: I017c2f83ce70545b9044e11ade7e5c15e8ca8a53