[Notifs] Replace `isPinned` boolean with a 3-option enum instead.

Currently, notifications can only be shown heads up if we decide the
notification should be heads up when it's added or updated.

With the status bar notification chips, notifications will *also* be
shown heads up if the user taps on the notification chip.

The status bar needs to have different behavior based on whether the
notification is HUNing because it was added/updated or because the user
tapped on the chip:
 - If added/updated like normal: Hide the clock, show just the new
   notification icon, hide the chips
 - If user tapped on chip: Show clock, show the chip, hide the
   notification icons

This CL replaces the `isPinned` boolean with a 3-option enum so that we
know *why* a notification is being shown heads-up. A future CL will use
PinnedStatus.PinnedDueToChip when the user taps on a chip and update the
status bar behavior accordingly.

Bug: 364653005
Flag: EXEMPT refactor
Test: Key notification CUJs work:
 - Receive notif while unlocked -> notif HUNs
 - Swipe up on HUN -> notif snoozed
 - Swipe left/right on HUN -> notif dismissed
 - Tap on HUN -> launches activity
 - Receive notif while locked with bypass enabled -> notif HUNs
 - Receive notif while locked with bypass disabled -> notif becomes part
   of stack
 - Enable notification cooldown -> notifs cool down

Test: atest BaseHeadsUpManagerTest HeadsUpManagerPhoneTest
Change-Id: Ic60a75e8651a02aa7d7b435f39a01c634332e0c0