On-device screenshot support.

This comes in the form of a command, `screenshot`, which
will read /dev/graphics/fb0 (in a manner very similar to
adb's framebuffer_service) and write to the specified PNG
file.

Additionally, dumpstate now accepts a -p flag (mnemonic:
"picture" or "png") that, when specified, will cause a
screenshot to be captured in the same directory as the
bugreport.

Future work: invoke `dumpstate -p` when the bugreport
keychord is pressed, giving users a convenient way to attach
screenshots to bug reports (or simply take screenshots at
all without developer tools).

Bug: 2216571 (and probably plenty of others)
Change-Id: I36afbc55a0308a7bc01112ef39c4c62777efb203