Bug 1267305 fixups

[nthomas-mozilla]

Fixes several issues with #16

• url prefs should be set to "about:blank" instead of "" to ensure no tab loads
• LaterRun pref minTime should be minimumHoursSinceInstall; minSessionCount should be minimumSessionCount
• LocalizablePreferences yields prefs set to (eg) data:text/plain,browser.laterrun.pages.test.url=https://www.mozilla.org/en-US/%APP%/%VERSION%/secondrun/?f=76&v=2. This causes LaterRun to reject the url. Swapping to Preferences avoids this, and lets us type the boolean and int prefs correctly
• LaterRun doesn't interpolate any variables like %VERSION% (bug 1270707) so we have to hardcode values in browser.laterrun.pages.test.url. Good thing we've only got one locale here