Add fallback state for missing GUI apps

[panDing19]

Problem

Some GUI tasks depend on project-shadow apps that may be absent from the recommended Android image, notably TestMall and Mattermost. The initializer previously failed the task when the package was missing, even though the fixture state can still be used by app-direct tools and graders.

Affected tasks

• TGUI05_contacts_birthday_calendar_gift (com.testmall.app)
• TGUI08_sms_complaint_helpdesk_mattermost (com.mattermost.rnbeta)

Fix

• Always write shadow app fixture state to /data/local/tmp/claw_gui_state/<package>/state.json.
• If the app package is installed, continue writing its app-private state path and relaunching it.
• If the app package is missing, warn and return success after writing the fallback state.
• Add a regression test proving missing packages do not trigger a failed initialization or app launch.

Tests

• uv run pytest tests/test_mobile_gui_init.py -q