Monitor BLE_SCAN_ALWAYS_AVAILABLE setting

Check for updates to the Bluetooth scanning setting by montioring
BLE_SCAN_ALWAYS_AVAILABLE instead of ACTION_BLE_STATE_CHANGED.

Bug: 273547029
Test: Disable main BT setting, disable Bluetooth scanning, verify that
CHRE receives a BT setting update, enable Bluetooth scanning, verify
that CHRE receives a BT setting update.
Test: Disable Bluetooth scanning, disable main BT setting verify that
CHRE receives a BT setting update, enable main BT setting, verify
that CHRE receives a BT setting update.

Change-Id: I147d8632a02d675cbd426108a6099e5dc887b8a7