wifi(implementation): Clear wifi event callback objects while stopping

This fixes a deadlock between WifiNative and HalDeviceManager
because it removes about 30 times callbacks of
IWifiEventCallback#onFailure() or onStop(). Such excessive callbacks
within a short period tend to make high probabilities of the deadlock.

Note: This is a side effect of how the framework is registering multiple
callbacks. But, unfortunately there is no HAL API to unregister
callback. So, this is somewhat of a workaround fix.

Bug: 144137870
Test: Verified that the HAL no longer accumulates callbacks from
framework.
Test: Device boots up and connects to wifi networks.
Change-Id: I441e2f815a0a49c3936615f06a65a6ed5366a628
Merged-In: I441e2f815a0a49c3936615f06a65a6ed5366a628