Add Android support

[fabricedesre]

This adds support for android targets. Changes needed are:

• use the android specific version of the MetadataExt trait.
• use libc::mode_t instead of hardcoding the type for file modes (it's a u16 on 32bit Android targets).

[eminence]

LGTM, thanks!

I'm not an expert in android development, so a quick question for you: do you have any recommendations on testing android in CI? Is running cargo check --target aarch64-linux-android sufficient? Or are there other targets that should be also checked?

[fabricedesre]

Yes, running cargo check --target aarch64-linux-android is a good way to verify the Rust parts, while not having to deal with the link phase which is more annoying because you need an android sysroot.

[eminence]

Ok great. I'll add that later. Thank you!

[eminence]

This has now been published on crates.io as v0.9.0. Thank you!