feat(spanner): Cleanup GcpFallbackChannel creation and enable by default #20277

Job	Run time
gapic-libraries-bom	18m 39s
lint	3m 24s
changes	4s
bulk-filter	1m 4s
windows	2m 32s
generation-config-check	1m 33s
enforcer	17m 43s
split-units (java-spanner, 8)	4m 46s
split-clirr (java-spanner)	2m 53s
split-dependencies (java-spanner)	3m 50s
split-units (java-spanner, 25)	7m 41s
split-units (java-spanner, 11)	5m 28s
split-units (java-spanner, 21)	5m 24s
split-units (java-spanner, 17)	5m 20s
units (8)	45s
units (17)	30s
units (25)	29s
units (11)	38s
units (21)	33s
conditional-required-check	4s
	1h 23m 20s

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