app_process: use LOCAL_MULTILIB := both

Use LOCAL_MULTILIB := both to build app_process64 instead of
duplicating the build rule.

Also causes PRODUCT_PACKAGES := app_process to install both
versions on 64-bit platforms, as the module names for both
app_process and app_process64 are now app_process.

Change-Id: Ia3c3265d15475771b0cae8bace53b8fbf351c186