ANDROID: binder: allow changing binder.impl during boot

Currently it is required that you set binder.impl directly on the kernel
commandline from the bootloader. This has the unfortunate consequence
that every class of device that wants to try Rust Binder has to have its
bootloader changed to accomodate that.

Previously, there was no way around that because Rust Binder was built
in to the kernel together with C Binder, and so the information really
had to be available on the kcmdline. There was no other choice than to
modify the bootloader. However, today we have moved Rust Binder to a GKI
module, which means that we don't need the value of binder.impl to be
set until rust_binder.ko actually gets loaded. Thus, allow binder.impl
to be changed via sysfs, as long as the change happens before
rust_binder.ko is loaded (or C Binder is otherwise used).

Note that anybody who can modify the flag via this mechanism could also
do so previously via the command line. It's just a lot easier to do it
with this change, as you no longer have to change the bootloader.

Bug: 442512016
Change-Id: I1819202ac1e08b6b89ac8c50d36a3f4dcad4aa3c
Signed-off-by: Alice Ryhl <aliceryhl@google.com>