Adjust VM size for Tuna devices

Related to bug report here:
https://code.google.com/p/android/issues/detail?id=40961

Not positive it will make a difference, running a test build now.

Change-Id: Ifad7e47a36d27626ffa48ef3613cd33969e56cc1
Signed-off-by: Zaphod <mwilson@acesappraisal.com>