Fix mdns client example shutdown sequence.

[bzbarsky-apple]

Shutting down the platform manager while the event loop is not fully shut down (which is the case when StopEventLoopTask is called from inside the event loop itself) is not supported.

Fixes #8917

[github-actions]

PR #25889: Size comparison from 2aefaa6 to c2b96af

Decreases (1 build for cc32xx)

platform	target	config	section	2aefaa6	c2b96af	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL	.debug_info	20310044	20310043	-1	-0.0

Full report (1 build for cc32xx)

platform	target	config	section	2aefaa6	c2b96af	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	645769	645769	0	0.0
			(read/write)	203848	203848	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197248	197248	0	0.0
			.comment	194	194	0	0.0
			.data	1480	1480	0	0.0
			.debug_abbrev	933102	933102	0	0.0
			.debug_aranges	87704	87704	0	0.0
			.debug_frame	301596	301596	0	0.0
			.debug_info	20310044	20310043	-1	-0.0
			.debug_line	2681024	2681024	0	0.0
			.debug_loc	2827360	2827360	0	0.0
			.debug_ranges	286352	286352	0	0.0
			.debug_str	3041144	3041144	0	0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	105953	105953	0	0.0
			.shstrtab	232	232	0	0.0
			.stab	204	204	0	0.0
			.stabstr	441	441	0	0.0
			.stack	2048	2048	0	0.0
			.strtab	380543	380543	0	0.0
			.symtab	257472	257472	0	0.0
			.text	537696	537696	0	0.0