Fetching upstream

[Description]
Preparing for introducing Compose, which requires Android Studio Canary,
which requires Gradle 7+

[Solution]
- Upgrade Gradle and Android Gradle Plugin
- Make necessary changes to accomodate the new Gradle and AGP APIs

[Testing]
- ./gradlew build
- manual testing of Android and iOS apps

[Description]
Jetpack Compose is going stable in July 2021, and we want to show folks
the easiest way to get started with KMM using Compose.

[Solution]
- Add Compose UI, and support SwipeRefresh using Accompanist.
- Remove XML layouts.

[Testing]
- ./gradlew :shared:build :app:build
- manual testing

[Summary]
The Loading UI is usually shown in conjunction with other UI, such as an
error view, or valid data. Consuming it with a when statement and
treating it like any other state may not make sense with Compose and
Swift UI. I tried a different approach here, giving the Loading state
the last valid DataState as well:
#6

[Fix]
In this PR, I'm using the classic NetworkBoundResource approach, where
the DataState is not a sealed class; instead, it contains nullable
versions of all possible states.

[Testing]
- `./gradlew :app:build`
- `./gradlew :shared:build`
- `xcodebuild -workspace ios/KaMPKitiOS.xcworkspace -scheme KaMPKitiOS
    -sdk iphoneos -configuration Debug build -destination name="iPhone 8"`
- manual testing