MemoryHeapBase: Save and binderize the offset

The offset that is used in the creation of the MemoryHeapBase must be saved, so
that it can be used to recreate the Heap when an IMemory object is passed
across process boundary through the binder.

Change-Id: Ie618fb5c0718e6711f55ed9235616fd801e648dc
Signed-off-by: Anu Sundararajan <sanuradha@ti.com>
Signed-off-by: Iliyan Malchev <malchev@google.com>