Implement "Hide non-clearable" and "Hide low-priority"

Non-clearable notifications show up in Hover and are not
dismissible by swiping. One has to wait for the "Evade notification"
timeout for the notification to be removed from the screen.
Example - Notification when USB debugging is connected.

Some apps like Google Now have a tendency to show low-priority
notifications rather frequently (like weather updates).

These configurable options allow one to hide low-priority as well
as non-clearable notifications from Hover. They'll be sent to
notification drawer directly.

Signed-off-by: Shubhang Rathore <shubhang92@gmail.com>