Auto generate dark mode previews and screenshots (#776)

With this change, composable previews and screenshots should be created with just:
```
@ElementPreviews
@Composable
fun MyViewPreview() {
    ElementPreview { 
        MyView()
    }
}
```

- Adds `@ElementPreviews` which is a shorthand for:
```
@Preview(name = "D")
@Preview(name = "N", uiMode = Configuration.UI_MODE_NIGHT_YES)
```
Should be used in connection with the now public `fun ElementPreview()` composable.

- Adds ElementPreviews to previewAnnotations in dangerfile
- Screenshots of night mode previews are now rendered with night mode
- Replaces `ElementPreviewLight` and `ElementThemedPreview` with `ElementPreview`
- Deprecates `ElementPreviewDark` which should be removed.
- Remaining usages of `ElementPreviewDark` are now ignored during screenshot tests

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
@@ -59,7 +60,11 @@ class ScreenshotTest {
     object PreviewProvider : TestParameter.TestParameterValuesProvider {
         override fun provideValues(): List<TestPreview> {
             val metadata = Showkase.getMetadata()
-            val components = metadata.componentList.map(::ComponentTestPreview)
+            val components = metadata.componentList.filterNot {
+                // skip all previews that have "dark" in the name because
+                // all previews get a dark mode screenshot automatically now
+                it.componentKey.contains("dark", true)
+            }.map(::ComponentTestPreview)
             val colors = metadata.colorList.map(::ColorTestPreview)
             val typography = metadata.typographyList.map(::TypographyTestPreview)
 
@@ -95,6 +100,10 @@ class ScreenshotTest {
                 ),
                 LocalConfiguration provides Configuration().apply {
                     setLocales(LocaleList(localeStr.toLocale()))
+                    // Dark mode previews have name "N" so their component is subfixed with "- 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}"
 }