increase HeapCapacity to 256MB in bootimage.cpp
increase HeapCapacity to 256MB in bootimage.cpp

This is necessary to avoid running out of memory when building
bootimages of certain apps with OpenJDK using ProGuard'ed code.

badlogic/avian,bigfatbrowncat/avian-pack.avian,getlantern/avian,joshuawarner32/avian,ucdseniordesign/avian,MaartenR/avian,dicej/avian,lostdj/avian,lostdj/avian,getlantern/avian,minor-jason/avian,bgould/avian,badlogic/avian,minor-jason/avian,lwahlmeier/avian,MaartenR/avian,joshuawarner32/avian,bgould/avian,getlantern/avian,dicej/avian,marcinolawski/avian,ucdseniordesign/avian,ucdseniordesign/avian,getlantern/avian,ucdseniordesign/avian,minor-jason/avian,badlogic/avian,lwahlmeier/avian,lwahlmeier/avian,bigfatbrowncat/avian-pack.avian,dicej/avian,lostdj/avian,marcinolawski/avian,dicej/avian,marcinolawski/avian,joshuawarner32/avian,minor-jason/avian,marcinolawski/avian,bgould/avian,MaartenR/avian,MaartenR/avian,bigfatbrowncat/avian-pack.avian,lostdj/avian,bigfatbrowncat/avian-pack.avian,joshuawarner32/avian,bgould/avian,badlogic/avian,lwahlmeier/avian