Support dpi specific aapt flags.

With this support, you can set dpi-specific aapt flags like:
$(LOCAL_PACKAGE_NAME)_aapt_flags_hdpi := --version-code 100 \
                                         --version-name "100
which will override the --version-code and --version-name set in the
base apk's LOCAL_AAPT_FLAGS.

Bug := 18388705

Change-Id: Ie79d7b1b6bbf7463bce51ddae04ba81384ec58dc