[mle] retain direct child cache entries on Addr Solicit Response TX

[abtink]

Previously, when a child was promoted to a router role, the leader upon sending "Address Solicit Response" would clear cache entries associated with the child's old RLOC16. This commit modifies the code to retain these cache entries when the sender is a direct child of the leader. By retaining the cache entries, they can be associated with the promoted router's new RLOC16 when the Link Advertisement is received.

[size-report]

Size Report of OpenThread

Merging #9956 into main(51ab865).

name	branch	text	data	bss	total
ot-cli-ftd	main	466848	760	66356	533964
	#9956	466848	760	66356	533964
	+/-	0	0	0	0
ot-ncp-ftd	main	436164	760	61568	498492
	#9956	436180	760	61568	498508
	+/-	+16	0	0	+16
libopenthread-ftd.a	main	235881	0	40302	276183
	#9956	235893	0	40302	276195
	+/-	+12	0	0	+12
libopenthread-cli-ftd.a	main	57342	0	8075	65417
	#9956	57342	0	8075	65417
	+/-	0	0	0	0
libopenthread-ncp-ftd.a	main	31857	0	5916	37773
	#9956	31857	0	5916	37773
	+/-	0	0	0	0
ot-cli-mtd	main	364808	760	51220	416788
	#9956	364808	760	51220	416788
	+/-	0	0	0	0
ot-ncp-mtd	main	347372	760	46448	394580
	#9956	347372	760	46448	394580
	+/-	0	0	0	0
libopenthread-mtd.a	main	158395	0	25182	183577
	#9956	158395	0	25182	183577
	+/-	0	0	0	0
libopenthread-cli-mtd.a	main	39775	0	8059	47834
	#9956	39775	0	8059	47834
	+/-	0	0	0	0
libopenthread-ncp-mtd.a	main	24737	0	5916	30653
	#9956	24737	0	5916	30653
	+/-	0	0	0	0
ot-cli-ftd-br	main	535864	768	130900	667532
	#9956	535880	768	130900	667548
	+/-	+16	0	0	+16
libopenthread-ftd-br.a	main	299372	5	104822	404199
	#9956	299384	5	104822	404211
	+/-	+12	0	0	+12
libopenthread-cli-ftd-br.a	main	70975	0	8099	79074
	#9956	70975	0	8099	79074
	+/-	0	0	0	0
ot-rcp	main	62248	564	20604	83416
	#9956	62248	564	20604	83416
	+/-	0	0	0	0
libopenthread-rcp.a	main	9542	0	5052	14594
	#9956	9542	0	5052	14594
	+/-	0	0	0	0
libopenthread-radio.a	main	18821	0	214	19035
	#9956	18821	0	214	19035
	+/-	0	0	0	0