fix(android): Declare test-snapshots as task output for cache compatibility (#5396)

* fix(android): Declare test-snapshots as task output for cache compatibility

The screenshot snapshot PNGs written by ScreenshotEventProcessorTest are
not declared as outputs of testDebugUnitTest. When the task result comes
from the Gradle remote cache, the test code never runs and the directory
is never created, so sentry-cli finds an empty folder and uploads nothing.

Declaring the directory as a task output ensures Gradle caches and
restores the snapshots on cache hits.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Format code

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Sentry Github Bot <bot+github-bot@sentry.io>