Add background color support to theme components

[mikemilla]

Summary

This PR adds granular background color support to the theme system, enabling consumers to style backgrounds of individual UI sub-components (message cells, bottom sheets, and info views) rather than only the root container.

Context

Relates to courier-react-native#46 (bump courier-android to 5.3.1) and courier-react-native#45 (bump Courier_iOS to 5.8.1). Those PRs bump the native SDK dependency versions in the React Native wrapper.

Changes

SDK library (android/)

• CourierInboxTheme: Added cellBackgroundColor: Int? property to control the background color of individual inbox message cells (previously hardcoded to ?android:colorBackground in XML)
• CourierPreferencesTheme: Added sheetBackgroundColor: Int? property to control the background color of the preference topic bottom sheet
• CourierStyles.InfoViewStyle: Added backgroundColor: Int? property to control the background color of info/empty/error state views
• MessageItemViewHolder: Wired up cellBackgroundColor to set the cell container background
• PreferenceTopicBottomSheet: Wired up sheetBackgroundColor to set the sheet root view background
• CourierInfoView: Refactored into a shared applyInfoViewStyle() method that applies InfoViewStyle.backgroundColor, font, and button styling

Example app (app/)

• Updated ComposeInboxActivity, StyledInboxFragment, and StyledPreferencesFragment to demonstrate the new background color properties

Usage

// Inbox with custom cell background
CourierInboxTheme(
    backgroundColor = Color.WHITE,
    cellBackgroundColor = Color.parseColor("#F5F5F5"),
    // ...
)

// Preferences with custom sheet background
CourierPreferencesTheme(
    backgroundColor = Color.WHITE,
    sheetBackgroundColor = Color.parseColor("#FAFAFA"),
    // ...
)

// Info view with custom background
CourierStyles.InfoViewStyle(
    font = CourierStyles.Font(),
    button = CourierStyles.Button(),
    backgroundColor = Color.parseColor("#F0F0F0"),
)

Testing

• Library compiles clean (./gradlew :android:compileDebugKotlin)
• All existing unit tests pass (./gradlew :android:testDebugUnitTest)