Fix VoiceTrigger placement for Android < 16

[palazik]

This change fixes a bug where VoiceTrigger was incorrectly placed on older Android versions. It introduces a modular VoiceTriggerFixPlugin that handles moving the VoiceTrigger application from the /product partition to /system_ext when the base Android version is less than 16.

To maintain compatibility with existing tests that mock internal SystemModifier methods, backward compatibility methods (stubs and proxies) were added to the SystemModifier class in unified_modifier.py.

The new plugin is automatically discovered and executed by the UnifiedModifier system. All tests, including the specialized test_voice_trigger.py, pass successfully.

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PR created automatically by Jules for task 5991654330523645076 started by @palazik

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