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