Optional day night preview annotation (#793)

Adds the `@DayNightPreviews` annotation that when used on a composable will:
- Display both a day mode and night mode preview in Android Studio.
- Produce both a day and night screenshot during screenshot testing.

The usage of this new annotation is optional, all the current previews continue to work without breakages.
New code can use, when appropriate, the new `@DayNightPreviews` annotation and replace the pattern using three `LightPreview/DarkPreview/ContentToPreview` functions with:

```
@DayNightPreviews
@Composable
fun MyScreenPreview(@PreviewParameter(MyStateProvider::class) state: MyState) {
    ElementPreview {
        MyScreen(
            state = state,
        )
    }
}
```

tests/uitests/src/test/kotlin/io/element/android/tests/uitests/ColorTestPreview.kt

@@ -38,5 +38,7 @@ class ColorTestPreview(
         )
     }
 
+    override val name: String = showkaseBrowserColor.colorName
+
     override fun toString(): String = "Color_${showkaseBrowserColor.colorGroup}_${showkaseBrowserColor.colorName}"
 }

tests/uitests/src/test/kotlin/io/element/android/tests/uitests/ComponentTestPreview.kt

@@ -25,5 +25,7 @@ class ComponentTestPreview(
     @Composable
     override fun Content() = showkaseBrowserComponent.component()
 
+    override val name: String = showkaseBrowserComponent.componentName
+
     override fun toString(): String = showkaseBrowserComponent.componentKey
 }

tests/uitests/src/test/kotlin/io/element/android/tests/uitests/ScreenshotTest.kt

@@ -38,6 +38,7 @@ import com.airbnb.android.showkase.models.Showkase
 import com.android.ide.common.rendering.api.SessionParams
 import com.google.testing.junit.testparameterinjector.TestParameter
 import com.google.testing.junit.testparameterinjector.TestParameterInjector
+import io.element.android.libraries.designsystem.preview.NIGHT_MODE_NAME
 import io.element.android.libraries.theme.ElementTheme
 import org.junit.Rule
 import org.junit.Test
@@ -95,6 +96,10 @@ class ScreenshotTest {
                 ),
                 LocalConfiguration provides Configuration().apply {
                     setLocales(LocaleList(localeStr.toLocale()))
+                    // Dark mode previews have name "N" so their component name contains "- N"
+                    if (componentTestPreview.name.contains("- $NIGHT_MODE_NAME")){
+                        uiMode = Configuration.UI_MODE_NIGHT_YES
+                    }
                 },
                 // Needed so that UI that uses it don't crash during screenshot tests
                 LocalOnBackPressedDispatcherOwner provides object : OnBackPressedDispatcherOwner {

tests/uitests/src/test/kotlin/io/element/android/tests/uitests/TestPreview.kt

@@ -21,4 +21,6 @@ import androidx.compose.runtime.Composable
 interface TestPreview {
     @Composable
     fun Content()
+
+    val name: String
 }

tests/uitests/src/test/kotlin/io/element/android/tests/uitests/TypographyTestPreview.kt

@@ -44,5 +44,7 @@ class TypographyTestPreview(
         )
     }
 
+    override val name: String = showkaseBrowserTypography.typographyName
+
     override fun toString(): String = "Typo_${showkaseBrowserTypography.typographyGroup}_${showkaseBrowserTypography.typographyName}"
 }