Fix reference to uncleared variable in dex_preopt_odex_install.mk

dex_preopt_odex_install.mk referenced $(built_module), which is only
set by some prebuilt modules, leaving old values when building
non-prebuilt modules.  Use $(LOCAL_BUILT_MODULE) instead, which is set
for all modules by base_rules.mk.

Test: manual
Change-Id: Iecf092f9869d5edda3671d0999987906781d023e