Build libcore without a dep on aapt.

Currently building libcore (i.e. to get the core Java libraries) has a
dependency on AAPT. This is problematic for some users that don't want to pull
in the entirety of frameworks/base, e.g. for Dalvik development.

It turns out that all definitions.mk is using aapt for in this case is 'aapt
add -k <jar> <dex>', which adds a file to a JAR while stripping path info.
This is functionally equivalent to 'zip -j', and this CL just makes that
switch.

Change-Id: I292ecaf9d0321018bb7d2e68a49b2cc81de98a20