fix: Create a single S2AChannelCredentials per application #12563

Job	Run time
showcase-clirr	3m 41s
build(8) for gapic-generator-java	11m 10s
lint	1m 3s
compatibility	1m 46s
build(8) except for gapic-generator-java	4m 48s
build(8) for showcase	4m 50s
build (17)	14m 48s
showcase (11)	10m 14s
build (11)	14m 21s
showcase (21)	10m 1s
build(21) except self-service clients	13m 14s
build(25) except self-service clients	12m 44s
unmanaged_dependency_check	7m 24s
gapic-generator-java-bom	2m 18s
showcase (17)	10m 9s
showcase (25)	10m 28s
dependency_analyzer_unit_test	22s
	2h 13m 21s

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