stability: fix bug which happens when toggle back and forth between UI-with-realtime-video-inference and other UI

[jeffzhou2000]

Purpose

• stability improvement: fix an unaccpetable bug which happens when toggle back and forth between UI-with-realtime-video-inference and other UI.
• remove ggml-jni-impl-external.cpp and remove function ggml_jni_bench_mulmat, this big source file and that function can be deprecated since 05/30/2025.

Todo

• the camera could display a blank screen when toggle back and forth between UI-with-realtime-video-inference and other UI. restart the APK is required to fix this issue.
• merge the logic in ggml-jni-context.cpp and realtime-video-recognition.cpp.