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: I1d87c3e394acc934af6f7770ea54ba265a1df838
update ota_from_target_files to handle mounting/unmounting for backupscript

backupscript should not be mounting/unmounting itself as it makes other
scripts have unexpected results (such as modelid_cfg, which expects /system
to be mounted)

instead have the ota script handle the mounting/unmounting

Change-Id: I94511f4147c624d975cb3ecbeaa8b0e98f63437c