xdsclient: fix unexpectedly large LoadReportInterval in initial load report request #15482

Job	Run time
vet-proto	34s
tests (tests, 1.24, -race)	6m 7s
tests (tests, 1.24)	5m 9s
tests (extras, 1.24)	2m 15s
tests (vet, 1.23)	1m 31s
tests (tests, 1.24, 386)	5m 20s
tests (tests, 1.23)	3m 48s
tests (tests, 1.24, arm64)	10m 39s
tests (tests, 1.24, -race, GRPC_EXPERIMENTAL_ENABLE_NEW_PICK_FIRST=false)	6m 17s
tests (tests, 1.24, arm64)	7m 7s
tests (extras, 1.24)	3m 0s
vet-proto	34s
tests (tests, 1.24, -race)	6m 7s
tests (tests, 1.24, 386)	5m 20s
tests (tests, 1.24, -race, GRPC_EXPERIMENTAL_ENABLE_NEW_PICK_FIRST=false)	6m 17s
tests (tests, 1.23)	3m 48s
tests (vet, 1.23)	1m 31s
tests (tests, 1.24)	5m 9s
	1h 20m 33s

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