[Spa] Clean up LiveData

This change removes the usage of and dependency on `LiveData` from the
SPA library.

Here's a breakdown of the changes:

*   **`LiveDataExt.kt` deleted:** This file contained a single
    Composable extension function, `observeAsCallback`, which wrapped
    `LiveData.observeAsState` to provide the `LiveData`'s value as a
    lambda. This utility is no longer needed.
*   **`build.gradle.kts` and `Android.bp` updated:** With the removal of
    the `LiveData` utility, the dependencies on
    `androidx.compose.runtime:runtime-livedata` and
    `androidx.lifecycle:lifecycle-livedata-ktx` are no longer required.
    They have been removed from both the Gradle and Soong build files to
    clean up the project's dependencies.

In essence, this is a refactoring that simplifies the library by
removing an unused feature and its associated dependencies, likely
favoring other reactive data holders like `StateFlow` for Jetpack
Compose integration.

Fix: 321163306
Flag: EXEMPT clean up
Test: m SpaLib
Change-Id: I4c1c892bb01c116284b589c1faf9959b7bfcf0b2