iot: extract boot parameter logic.

Currently the boot parameters are tied pretty tightly to the bootaction
functionality, but volume and brightness need to be set on the
bootanimation regardless of whether there's a bootaction or not.

Extract boot parameters into a separate class to make it easier to apply
volume/brightness in a future CL.

Bug: 65462981
Test: Manual test, can succesfully read params on boot.
Change-Id: I32daad64cb8aab39fcd0ca17503218e0605ccd27
(cherry picked from commit f78561e7bbe580d0f0dbca7a615c575973ef6ce4)