`File::lock` does not work on Android

[RalfJung]

Currently, our lock method just always errors on Android.

However, the libc crate does expose flock on Android, and cargo-bins/cargo-binstall#2330 sounds like there is support for flock on Android with fs4, at least for some Android versions.

We should figure out which Android versions have flock, and whether we can use that for the standard library API.

(Also see #146312 which is a similar issue for Illumos.)

Cc @chriswailes @maurer @mgeisler @tgross35