Keep all Soong command line arguments in a single structure

Define a structure android.CmdArgs and keep all soong_build command line
arguments used to build a configuration in it. Some of them related to BazelMode
were previously kept in Blueprint's bootstrap.Args where they do not belong.
This simplifies the code and lays the gound work for future simplifying changes.

Test: treehugger
Change-Id: I995871f4e6b0e95cc9df65696ad2020df221d8c8