fix: toggle map color scheme from system state

[himu-gupta]

Summary

• Make the first sample dark-mode toggle switch away from the effective system scheme.
• Preserve explicit LIGHT and DARK toggling behavior.
• Add unit coverage for light, dark, and FOLLOW_SYSTEM transitions.

Root cause

The sample initializes the map with FOLLOW_SYSTEM, but its first button press always selected DARK. On an API 37 emulator already using dark theme, that is visually DARK to DARK, so the map appears not to react even though Compose state updates. The library runtime mapColorScheme propagation is unchanged.

Verification

• git diff --check
• Focused unit tests added in BasicMapActivityTest
• Local Gradle test/build: blocked by the Codex host sandbox denying the Gradle file-lock socket
• API 37 before/after screenshots: blocked because no Maps API key is available in the environment

CI is expected to run the repository build, unit tests, screenshot tests, lint, and instrumentation suite.

Fixes #917

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