[Android] Respect Retry-After for preload throttling

[markmur]

What changes are you making?

• Parse Retry-After delay-seconds and HTTP-date response headers.
• Add a managed preload throttle policy by default: a 429 with a valid Retry-After value returns a throttled failure and suppresses explicit preload calls until the delay elapses.
• Add a passthrough policy that returns the existing HTTP failure with retryAfterSeconds metadata and applies no cooldown.
• Expose retryAfterSeconds on presentation CheckoutException values without changing presentation behavior.
• Preserve the existing Java constructors, document the policies, and update the public Android API baseline.

Checkout Kit never retries a throttled preload automatically, and preload cooldowns never block presentation.

Before you merge

• I have added tests to support my implementation.
• I have updated the Android README.

Focused parser, configuration, preload, presentation, and Java interop tests pass. The Android public API check and library Detekt pass. The aggregate Android formatter could not configure the demo app because generated local environment configuration is absent from the isolated worktree; library formatting checks pass.

[markmur]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• [Android] Respect Retry-After for preload throttling #661 👈 (View in Graphite)
• [Swift] Respect Retry-After for preload throttling #660
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