This CL adds generateJsonModuleGraph bazelBuildMode and ensures that it
returns Soong early, before Kati and Soong metrics collection begin
(which causes errors, since they're looking for files that Soong in json
dump mode did not write).
Test: TH
Test: rm -rf out && SOONG_DUMP_JSON_MODULE_GRAPH=/tmp/soong.json m nothing
Change-Id: I264eadb3b3b0cd6b6e7a65adc7b39bb1c01ca136
dd9725c177