art: fix host dex2oat runtime args

The runtime arguments need to be prefixed with -Xmx or -Xms.
This only worked before because DEX2OAT_XMS, etc. were blank
due to bugs in build/, which resulted in --runtime-arg --runtime-arg
and anything starting with -- as the argument to --runtime-arg is
silently ignored.

Change-Id: I18607a6fd8d0004466404045f8dc4495eb6ab275
3 files changed