Split envsetup.sh PATH setting into lunch-dependent and lunch-independent values

This gets b (and other scripts that are coming soon) into the path without running lunch.

Also gets rid of the make dumpvar ANDROID_BUILD_PATHS variable, used to set a few more paths,
so that the setting is all in one place, now that the get_abs_build_var is cached.

Test: Lots of manual checks of before and after combinations
Change-Id: I745168c43ccf2fef4cdd22657fd087f1ad07e510