chore(client-ffi): tweak uniffi settings (#10665)

As far as I can tell, the `async_runtime` config option doesn't exist in
UniFFI, hence we remove that.

Whilst going through the UniFFI docs, I also noticed that there is a
specific flag about Android that we can toggle on. Effectively, this
uses the shared
[`SystemCleaner`](https://developer.android.com/reference/android/system/SystemCleaner)
instead of a per-thread one which is supposed to be more performant.

Finally, using immutable records seems like a good idea as mutating any
FFI-originated field is not going to be reflected in connlib's state.
Preventing that at compile-time has a good chance of reducing bugs.

rust/client-ffi/uniffi.toml

@@ -4,13 +4,9 @@
 # Enable experimental features for better async support
 experimental_sendable_value_types = true
 
-# Configure async runtime for Swift
-# This enables proper async/await bridging from Rust to Swift
-async_runtime = "tokio"
-
 # Generate proper Swift concurrency annotations
 generate_immutable_records = true
 
 [bindings.kotlin]
-# Kotlin configuration (already working)
-async_runtime = "tokio"
+android = true
+generate_immutable_records = true