FROMLIST: binder: move binder_alloc to separate file

(from https://patchwork.kernel.org/patch/9817753/

)

Move the binder allocator functionality to its own file

Continuation of splitting the binder allocator from the binder
driver. Split binder_alloc functions from normal binder functions.

Add kernel doc comments to functions declared extern in
binder_alloc.h

Change-Id: I8f1a967375359078b8e63c7b6b88a752c374a64a
Signed-off-by: Todd Kjos <tkjos@google.com>