Fix handler cleanup for memory leaks

[callebtc]

Summary

• ensure `PosUiCoordinator` clears pending callbacks when stopping so the activity reference is not leaked through its `Handler`
• make `BalanceCheckActivity` remove pending messages in `onDestroy()` so foreground-dispatch callbacks do not leak the activity
• documented lint/test attempts (blocked locally because Android SDK is not configured on this runner)

Testing

• ./gradlew lint (fails: SDK location not found on runner)
• ./gradlew test (fails: SDK location not found on runner)

[callebtc]

utACK