le_scanning_manager: Avoid sending double enabled commands

In some cases OnResume may be called while Scan is already started.
This patch make sure the scan_on_resume_ flag is reset properly so
le_scanning_manager won't be sending double enabled commands to the
controller.

Bug: 298268010
Test: m
Test: atest avatar:AshaTest#test_pairing_dual_device              # 20x
Test: atest avatar:AshaTest#test_disconnect_initiator_dual_device # 20x
Test: atest 2 new unit tests
Test: bluetooth_AdapterSRHealth.sr_while_discovering.floss  # 100x
Test: Manually start scan and then trigger LE connection on Chromebook;
      The scan is paused and resumed correctly.
Change-Id: I7d257f28024009c27498158305080e42560a9e49