Fix adv instance validity check during suspend and resume of resolving list

Fix adv instance validity check during suspend and resume of resolving list

When BT is shutdown, multi advertisement instance pointer is made
null by BleAdvertisingManager::CleanUp function.
Any subsequent BleAdvertisingManager::Get function call made at
this point(eg during suspension and resumption of resolving list
activity)leads to a crash as the instance pointer is null.
Hence, use BleAdvertisingManager::IsInitialized function instead
of BleAdvertisingManager::Get function to check the validity of
multi advertisement instance.

Bug: 67325491
Change-Id: Ibbae7fa546bb52494be78eaff6d4b7b15614ab15