[Gen4] fixes BLE race condition while scanning

[XuGuohui]

Problem

Imagine the following scenario:

1. threadX: User app starts BLE scanning
2. BLE event thread: User app calls BLE.stopScanning() to stop scanning in scan result callback
3. [ threadX: BLE HAL acquires s_bleMutex ] in stopScanning() to try calling le_scan_stop() to stop scanning
4. [ BLE event thread: acquires sBleEventMutex ] since a new scan result is notified and BLE.stopScanning() is called again, which will try acquiring s_bleMutex, suspending...
5. threadX: continues to run and try acquiring sBleEventMutex before calling le_scan_stop(), suspending...
6. deadlock happens

Solution

Do not notify scan results once BLE.stopScanning() is called.

Steps to Test

user/tests/wiring/ble_scanner_broadcaster

Example App

N/A

References

N/A

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Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)