Allow DIST_DIR to be read via dumpvars

DIST_DIR is controlled by soong_ui, and the make dumpvars code no longer
has access to it. So instead, handle some dumpvar requests inside
soong_ui itself.

Test: get_build_var DIST_DIR
Test: get_build_var OUT_DIR
Test: get_build_var BUILD_SYSTEM
Test: build/soong/soong_ui.bash --dumpvars-mode --vars="DIST_DIR
BUILD_SYSTEM"
Change-Id: Id3bcb8b0748db67c83d0e42d1ae5be564c5eb2f7