binder: update MemoryHeapIon

* Update from ODROID-XU 04212014 BSP

Change-Id: Ifc2664bcde37a71d855e05e7c9e50288a4508892
reduce PB size from 2MB to 512KB

this allows us to enable h/w acceleration on low-end
devices while keeping memory usage down.

Bug: 6557760
Change-Id: I8af2de3038dc2579360b8b73aa452cb7a0e506a9