feat(spanner): Cleanup GcpFallbackChannel creation and enable by default #18687

Job	Run time
gitignore_correctness	59s
owlbot-py	1m 6s
group_id_check_for_maps_libraries	2m 34s
gapic-bom	1m 36s
owlbot-yaml	1m 1s
package_name_check	56s
generate-readme	1m 0s
root-pom	1m 37s
	10m 49s

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