ANDROID: rust_binder: choose driver with kcmdline flag

Makes it possible to enable both drivers at the same time and pick which
driver to use at boot by passing a kcmdline flag.

This is not intended to be upstreamed, but will be used to dogfood the
Rust Binder driver.

See also aosp/2908345 and gpar/802132 for configuring the bootloader to
send the kcmdline flag.

Bug: 278052745
Test: Verified that passing -extra_kernel_cmdline='binder.impl={c|rust}' to launch_cvd will boot with the chosen driver
Change-Id: I9d0b32ab9d76a9327d77fb60c1d984b3adaa1387
Signed-off-by: Alice Ryhl <aliceryhl@google.com>