Use CreateCodegenMetrics in api_bp2build as well

The internals of CodegenMetrics was updated in aosp/2276671. Use the
new API to initialize an empty object to prevent a null pointer
exception in api_bp2uild.

Test: m api_bp2build
Change-Id: Iad9c38b9881da896171f1c6d4e49d4875acfaab8