fix: make MobSFScanner import conditional for ARM64 compatibility

- Add try-except block to conditionally import MobSFScanner in modules/android/__init__.py
- Allows Android worker to start on ARM64 without MobSF dependencies (aiohttp)
- MobSF activity gracefully skips on ARM64 with clear warning message
- Remove workflow path detection logic (not needed - workflows receive directories)

Platform-aware architecture fully functional on ARM64:
- CLI detects ARM64 and selects Dockerfile.arm64 automatically
- Worker builds and runs without MobSF on ARM64
- Jadx successfully decompiles APKs (4145 files from BeetleBug.apk)
- OpenGrep finds security vulnerabilities (8 issues found)
- MobSF gracefully skips with warning on ARM64
- Graceful degradation working as designed

Tested with:
  ff workflow run android_static_analysis test_projects/android_test/ \
    --wait --no-interactive apk_path=BeetleBug.apk decompile_apk=true

Results: 8 security findings (1 ERROR, 7 WARNINGS)

backend/toolbox/modules/android/__init__.py

@@ -19,7 +19,13 @@ Modules for Android application security testing:
 # Additional attribution and requirements are provided in the NOTICE file.
 
 from .jadx_decompiler import JadxDecompiler
-from .mobsf_scanner import MobSFScanner
 from .opengrep_android import OpenGrepAndroid
 
-__all__ = ["JadxDecompiler", "MobSFScanner", "OpenGrepAndroid"]
+# MobSF is optional (not available on ARM64 platform)
+try:
+    from .mobsf_scanner import MobSFScanner
+    __all__ = ["JadxDecompiler", "MobSFScanner", "OpenGrepAndroid"]
+except ImportError:
+    # MobSF dependencies not available (e.g., ARM64 platform)
+    MobSFScanner = None
+    __all__ = ["JadxDecompiler", "OpenGrepAndroid"]