Capture the build command that executed the build system.

We need to capture the user build command to understand what additional
arguments is passed in to Soong. This data is useful to know which
targets are being executed and have statistics on top build targets
are specified.

Bug: b/169453712
Test: ran "m blueprint_tools", "m nothing", "lunch 1" and checked
      in soong_metrics proto.
Change-Id: I7ab94dcf037211f1abffe6afd012b03ffbf99756