Build - Use less heap space for dex on 32-bit build hosts

Run dex with -JXmx1024M instead of 2048M if running on a 32-bit host.
Surprisingly this appears to also work for framework and services; if
not, a little fine-tuning will be needed.

This does not change anything for non-32-bit hosts.

Change-Id: I6997fcac376fb5638cd77ad7845d298be137fdad
Signed-off-by: STELIX <ssspinni@gmail.com>