Find a better way to shutdown server and stop event loop for OTA Linux platform

[carol-apple]

Problem

This is to track the comment made at #16482 (comment)

Specifically, we added an OTARequestorInterface::Shutdown as a workaround for Linux/Darwin platforms to shut the server and stop event in order to apply (execv) the new image.

Proposed Solution

Call DeviceLayer::PlatformMgr().HandleServerShuttingDown() and DeviceLayer::PlatformMgr().StopEventLoopTask() directly from the platform impl of OTAImageProcessor.