Fix channel store requests.

Fixes plexinc/roku-client#28, among other things.

It turns out that we really can't make a single channel store request, or make
two simultaneously. So we're making them serially now. We're also exempting
the old firmware, since we can't figure out if the user was grandfathered, and
sideloaded channels.
Remove pref rewriting code that isn't needed anymore.
Extend the trial period on first run.
Remember that the channel has been purchased in the registry.

This should allow purchased channels to continue to work while disconnected
from the internet. If, somehow, the channel store request comes back and it
wasn't purchased, that will still take precedence.
my.plexapp.com -> plex.tv
Fix a crash when (broken) channels don't return a title.
Add clock display pref, fixes plexinc/roku-client-public#281.
Bump version to 2.8.4.

2.8.3 was only released in Japan. Or maybe I misremembered the last build
number and told Roku on the phone that they should expedite 2.8.4, and now
don't feel like calling them again to explain that I meant 2.8.3.
Fix clock display at 12pm.