[routing-manager] include Stub Router flag in emitted RAs by BR (#9486) #1428

Job	Run time
expects	32m 8s
packet-verification-1-1-on-1-3	21m 8s
packet-verification-low-power	7m 58s
thread-1-3-gcc-m32	15m 43s
thread-1-3-gcc-m64	15m 17s
thread-1-3-posix	12m 34s
thread-1-3-clang-10-m32	14m 36s
thread-1-3-clang-10-m64	13m 16s
upload-coverage	36s
delete-coverage-artifacts	10s
	2h 13m 26s

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