Fix SOONG_DUMP_JSON_MODULE_GRAPH on a fresh checkout.

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