fix: flush Android writes before emulator shutdown

[janicduplessis]

Description

stim stop and the other centralized Android teardown paths kill a running emulator immediately. Guest writes made just before shutdown can be lost, including a freshly installed APK.

Fixes #283

Solution

Flush the guest filesystem with adb shell sync before emu kill. A sync failure warns on stderr but does not block shutdown, preserving cleanup semantics. The helper is reached only after Stim re-resolves a running AVD as owned.

Test plan

• Focused Android teardown suites: 99 tests passed; temporarily removing the sync call made the order-sensitive regression test fail.
• Full checks passed: format, lint, build, typecheck, 3,501 unit tests, and 20 end-to-end tests.
• Real adb shell sync argument shape verified; persistence was not re-measured on a booted AVD because none was connected.