Floss: suspend and resume LE scanning

This patch handles the system suspend and resume for LE scanning.
- It pauses the scanning when the system is suspending.
- It resumes the scanning when the system is resuming.
- It takes care of both LE scanning and LE offload.
- It prevent APIs from re-enabling by returning the busy status.
- It calls the suspend mode change callbacks if any.

Note: We can't simply disable the LE scanning. When a filter is
offloaded with the MSFT extension and it is monitoring a device, it
sends a `Monitor Device Event` to indicate that monitoring is stopped
and this can cause an early wake-up. Until we fix the disable + mask
solution, we must remove all monitors before suspend and re-monitor
them on resume.

Bug: 269578912
Tag: #floss
BYPASS_LONG_LINES_REASON: Bluetooth likes 120 lines
Test: Manual - Tested with btclient
  bluetooth> floss enable
  bluetooth> le-scan register-scanner
  bluetooth> le-scan start-scan 1
  (... many ScanResult data showing up ...)

  The following suspend command should not be interrupted prematurely.
    $ powerd_dbus_suspend --wakeup_timeout=20;
  The /var/log/power_manager/powerd.LATEST displays something like
    INFO powerd: [dark_resume.cc(44)] Wake not triggered by user

  The btclient console should also display scan suspend mode change
  callback information such as Suspending, Suspended, Resuming, and
  Normal correctly.

Change-Id: I60ed8fc0afee49f2ef95f59cb388ca1ddfc7bf1f