Use Vec::as_mut_ptr to obtain a buffer for __system_property_get.

[jimblandy]

Use the idiom recommended by the Nomicon for allocating a buffer into which C code will write data:

https://doc.rust-lang.org/nomicon/ffi.html#creating-a-safe-interface

The current code is almost certainly fine in practice, but it doesn't follow the letter of the law. The documentation for CString::into_raw says:

The pointer which this function returns must be returned to Rust and reconstituted using CString::from_raw to be properly deallocated.

https://doc.rust-lang.org/std/ffi/struct.CString.html#method.into_raw

[jimblandy]

I haven't tested this code - I haven't set up an android development environment yet.

[nical]

LGTM, tested on my phone.