[res] Zero big buffer memory on backing up

When a memory chunk gets returned into BigBuffer, it may have
some data written already. BigBuffer is supposed to give out
zeroed memory, so BackUp() needs to zero it as well

Bug: 336758568
Bug: 342579978
Test: unit test + bundletool on the attached aab
Flag: EXEMPT bugfix
Change-Id: I8ecd60e84dbe16570a92d82370d1633af72599c8