[DON-2499] Fix AGP 9.0 compatibility issues for unit tests and build

- Add Robolectric SDK 35 configuration to work around Java 21 requirement for SDK 36
- Create screenshots flavor-specific BackpackDemoTheme to resolve missing class issue
- Add screenshots flavor theme resources needed for compilation
- Remove broken sourceSets configuration for screenshots flavor

Fixes lint check failure, unit test failures, and screenshot test compilation

Co-Authored-By: Claude <noreply@anthropic.com>

Co-authored-by: Claude <noreply@anthropic.com>

Author: 2 people

app/build.gradle.kts

@@ -76,12 +76,6 @@ android {
         }
     }
 
-    sourceSets {
-        getByName("screenshots") {
-            java.directories.add("src/internal/java")
-            res.directories.add("src/internal/res")
-        }
-    }
 
     testOptions {
         unitTests {

app/src/screenshots/java/net/skyscanner/backpack/demo/BackpackDemoTheme.kt

@@ -0,0 +1,38 @@
+/**
+ * Backpack for Android - Skyscanner's Design System
+ *
+ * Copyright 2018 - 2026 Skyscanner Ltd
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package net.skyscanner.backpack.demo
+
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.text.font.Font
+import androidx.compose.ui.text.font.FontFamily
+import androidx.compose.ui.text.font.FontWeight
+import net.skyscanner.backpack.compose.theme.BpkTheme
+
+@Composable
+fun BackpackDemoTheme(
+    content: @Composable () -> Unit,
+) {
+    BpkTheme(
+        fontFamily = FontFamily(
+            Font(R.font.skyscanner_relative_android_book),
+            Font(R.font.skyscanner_relative_android_bold, weight = FontWeight.Bold),
+        ),
+        content = content,
+    )
+}

app/src/screenshots/res/values/themes.xml

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+
+    <style name="AppThemeBase" parent="BpkTheme">
+        <item name="android:windowBackground">@color/bpkCanvas</item>
+        <item name="bpkFontFamilyBase">@font/skyscanner_relative_android_book</item>
+        <item name="bpkFontFamilyEmphasized">@font/skyscanner_relative_android_bold</item>
+        <item name="bpkFontFamilyBlack">@font/skyscanner_relative_android_black</item>
+
+        <item name="android:lineSpacingMultiplier">1</item>
+    </style>
+</resources>

app/src/test/resources/robolectric.properties

@@ -0,0 +1 @@
+sdk=35