Fix debug flag ordering

art_debug_defaults needs to come first in the defaults list so that its
prepended flags come after the art_defaults flags.
-Wno-frame-larger-than= also needs to be inside a target block to match the
way the -Wframe-larger-than= flags are inserted by art.go so that they
come after.

Change-Id: I17c191ec9dcc393db1e0805cd058c768e4d09400