ANDROID: ashmem_rust: Add is_ashmem_file()

The minidump module iterates over open files and VMAs for each process
and reports information that is useful for debugging. A useful piece of
information is what ashmem files are currently referenced either via
a userspace memory mapping or a file descriptor. Therefore, add
is_ashmem_file() to aid in identifying those references.

Unfortunately, the logic that exports Rust symbols to other kernel
modules only considers the crates under the rust/ directory at the root
of the kernel repository. Since there's no support to include symbols
from crates defined outside of that repository, add
ashmem_rust_exports.c to export is_ashmem_file().

Changes from older versions of this patch:

- Reworded commit message for clarity.
- Implemented is_ashmem_file() in Rust, since android16-6.12+ will use
  Rust for ashmem.

Bug: 193397560
Change-Id: I5b7816ad4775e5cf2c4f41c28b1c8dacc2c85b7e
Signed-off-by: Isaac J. Manjarres <isaacmanjarres@google.com>