Keyguard: Modify the effect of unread number

The unread number text size is defined in pixels, so it could not
adapt to different resolution devices well.

Modify the effect of unread number to be consistent with Launcher.
Define the unread number text size in dp and convert it in pixels.
Show "999+" when the unread number is more than 4 digits.
Update unread number background size and color.

CRs-Fixed: 698610

Change-Id: I289431d627951c05d7b874fb6e1540086b572462