floss: Defer unregister_scanner if it comes on suspending/ed

In the current implementation unregister_scanner is allowed even on
suspending/ed so the call from users could clash with Floss's suspend
process, and could potentially generate unexpected HCI commands/events
when the system is suspended. On resume, Floss may try to resume the
scanners that are already unregistered, the out-of-sync scanners info
could potentially cause more issues.

This patch adopts the same approach of Advertiser which defer the
unregistration after resume. Also aligns the suspend state check
between Scanner and Advertiser.

Bug: 288225857
Tag: #floss
Test: mmm packages/modules/Bluetooth
Test: manual verified cross device features + suspend resume on Trogdor
Flag: EXEMPT, Floss-only changes
Change-Id: I4a6327e5b2770ad06c90ca75b8a761e4261d36ab