Floss: Define suspend interface for LE Scan

This defines interfaces needed to implement suspend/resume for LE Scan:
* Internal methods `scan_enter_suspend` and `scan_exit_suspend` to be
  called by suspend module.
* StartScan and StopScan returning BtStatus to indicate the request
  being blocked in suspend mode.
* `SuspendMode` property so that clients can optionally wait for suspend
  to be over rather than retrying arbitrarily.

Bug: 224603540
Tag: #floss
Test: Manual - Build Floss on Linux

Change-Id: Iaaa4dc362d30de491616e9279e6a9d9a9fd35812