CameraMetadata: fix metadata alignment issue

When camera metadata is passed through binder interface, there is no
guarantee the destination address of the metadata copy is aligned to
the alignment boundary required by metadata copy, which could cause
metadata validation fail. this change aligns the start address of the
metadata copy destination blob to address this issue.

Bug: 12010193
Change-Id: I540c6b4c484fe87a1d625a362310f33a309c1772