ANDROID: rust: Use From instances for JSON encoder

This makes it easier to deal with nested objects or arrays.

This is Android specific because Rust KCFI is incomplete, and we are
using the binder driver as a staging ground to fix it. This patch is
intended to go upstream once the compiler behaves properly.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
Bug: 316623888
Change-Id: I1eb623bfa522fd46fe2850278dce78b7c4e2cdf1