FFs can now be toggled in release builds too (#3101)

- Removed `StaticFeatureFlagProvider`.
- Always provide `PreferencesFeatureFlagProvider`.
- For the default values of feature flags, use a lambda with a `BuildMeta` parameter so we can customize the return value based on its data.

samples/minimal/build.gradle.kts

@@ -30,6 +30,10 @@ android {
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"
     }
 
+    buildFeatures {
+        buildConfig = true
+    }
+
     buildTypes {
         release {
             isMinifyEnabled = false