#1956 fix emergency kill timestamp source

[keymapper-ai]

Summary

• switch system bridge emergency-kill hold detection to monotonic elapsed time in the evdev JNI observer
• stop using evdev event timestamps to decide power-button hold duration, preventing false emergency kills from timestamp drift
• add regression tests covering threshold behavior and timestamp-drift scenarios

Test plan

• cargo check -p evdev_manager_jni (from foss/evdev/src/main/rust/evdev_manager)
• run full Android/Gradle build in CI

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