Individually track vis duration of unseen notifs

Previously, if the user has been on the keyguard for 5 seconds, then we
mark all unseen notifications as seen when the device is unlocked, even
if some notifications were not present for the entire (or any of the) 5
second visible duration.

We now track for each unseen notification that since it has been posted
/ the device was locked, that the user has been on the keyguard for 5
seconds. When a notification has reached this threshold, it is marked as
"seen on lockscreen", and will be marked as seen for real once the
device is unlocked.

Note that if the notification is updated, the 5 second threshold is
reset.

REVERT^2: In order to reduce potential jank, we now only start tracking
seen notifications when locked if the setting is enabled.

Fixes: 277616032
Test: atest KeyguardCoordinatorTest
Change-Id: Iec2fdc977157bda85b41c5c64bfcce6d2ec9fd7f