android: binder: Move buffer out of area shared with user space

Binder driver allocates buffer meta data in a region that is mapped
in user space. These meta data contain pointers in the kernel.

This patch allocates buffer meta data on the kernel heap that is
not mapped in user space, and uses a pointer to refer to the data mapped.

Also move alloc->buffers initialization from mmap to init since it's
now used even when mmap failed or was not called.

Bug: 36007193
Change-Id: I691d522f46388b00400615c8bc55f0dceee821c8
Signed-off-by: Sherry Yang <sherryy@android.com>