[Catalyst] Update SettingsLib/Ipc/README.md

allows flexible threading model on top of the

[Kotlin coroutines](https://kotlinlang.org/docs/coroutines-overview.html).

## Caveats

IPC request data is transmitted with `Bundle` (see `MessageCodec`),

-   Bundle keys should not be changed to avoid breaking backward compatibility.

-   If strong backward compatibility is required, consider using serialization

    framework (e.g. Protobuf) and marshall/unmarshall with byte array.

-   When parcelable is used (e.g. `Bundle.putParcelable`), the class name must

    be the same across apps. Thus add `@Keep` annotation to the parcelable class

    if Proguard is enabled.

## Usage

The service provider should extend `MessengerService` and provide API

import android.content.Context

import android.content.Intent

import android.os.Bundle

import kotlinx.coroutines.runBlocking

class EchoService :

  MessengerService(

    listOf(EchoApiImpl),

    PermissionChecker { _, _, _ -> true },

  )

class EchoService : MessengerService(listOf(EchoApiImpl()), PermissionChecker { _, _, _ -> true })

class EchoServiceClient(context: Context) : MessengerServiceClient(context) {

  override val serviceIntentFactory: () -> Intent

    get() = { Intent("example.intent.action.ECHO") }

  private val echoApi = EchoApi()

  override val serviceIntentFactory = { Intent("example.intent.action.ECHO") }

  fun echo(data: String?): String? =

    runBlocking { invoke(context.packageName, EchoApi, data).await() }

  suspend fun echo(data: String?): String? = invoke(context.packageName, echoApi, data).await()

}

object EchoApi : ApiDescriptor<String?, String?> {

open class EchoApi : ApiDescriptor<String?, String?> {

  private val codec =

    object : MessageCodec<String?> {

      override fun encode(data: String?) =

        Bundle(1).apply { putString("data", data) }

      override fun encode(data: String?) = Bundle(1).apply { putString("data", data) }

      override fun decode(data: Bundle): String? = data.getString("data", null)

    }

}

// This is not needed by EchoServiceClient.

object EchoApiImpl : ApiHandler<String?, String?>,

                     ApiDescriptor<String?, String?> by EchoApi {

class EchoApiImpl : EchoApi(), ApiHandler<String?, String?> {

  override suspend fun invoke(

    application: Application,

    callingPid: Int,