ci: restore sdk-platform-java jobs #1210

Job	Run time
filter	1m 4s
build(8) except for gapic-generator-java	3m 39s
build(8) for gapic-generator-java	11m 26s
build(25) except self-service clients	12m 48s
build(8) for showcase	10m 11s
build(21) except self-service clients	10m 42s
showcase-clirr	5m 22s
unmanaged_dependency_check	9m 33s
build (11)	13m 38s
build (17)	13m 46s
dependency_analyzer_unit_test	1m 19s
gapic-generator-java-bom	3m 19s
showcase (21)	12m 50s
showcase (11)	13m 20s
showcase (17)	14m 11s
showcase (25)	13m 1s
	2h 30m 9s

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