perf(watcher): cut redundant work from each session ingest run

[0xbrayo]

Part of #333.

Session ingest runs hourly, on every app open and on widget refresh, and the first run after install covers all of the usage history Android retains. Each run did avoidable work:

• App labels: SessionUtils.getAppName (a PackageManager binder call) ran once per session, even though most sessions belong to a handful of apps.
• Event passes: queryEvents was called and fully iterated twice, once for sessions and once for unlocks, allocating a new UsageEvents.Event per entry.
• Bucket listing: createBucketHelper serializes every bucket via getBuckets() to check whether one exists, and ran twice per run.

Change

• parseEventsIntoSessions caches app labels for the duration of the run.
• New SessionParser.parseSessionsAndUnlocksSince(sessionsSince, unlocksSince) reads sessions and unlocks in one pass, reusing a single Event. It takes separate lower bounds so it composes with fix(watcher): resume unlock ingest from the unlock bucket's last event #335, which gives unlocks their own cursor. The now-unused parseUsageEventsSince and parseUnlockEventsSince are removed.
• RustInterface.createBucketsHelper(map) ensures several buckets with one listing. createBucketHelper delegates to it.

Notes

• Touches the same functions as fix(watcher): resume unlock ingest from the unlock bucket's last event #335, fix(parser): end sessions at screen-off instead of dropping long ones #336 and fix(watcher): count the in-progress foreground session #337. Whichever lands later needs a small rebase.

Testing

• ./gradlew :mobile:testStandardDebugUnitTest passes.
• The changed paths need UsageStatsManager and the native library, so this has no new unit test.

[greptile-apps]

Confidence Score: 5/5

[Medium impact] The PR appears safe to merge; no blocking issue was found.

Summary

This PR cuts repeated work when saving app sessions and screen unlocks.

• Session ingest gathers app sessions and unlocks in one event pass.
• Sessions from the same app reuse one app-name lookup.
• Session ingest checks both required buckets with one listing.

No actionable new issues were found. Tests were not rerun.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Start ingest run] --> B[List buckets once]
    B --> C[Create missing session and unlock buckets]
    C --> D[Read last stored session]
    D --> E[Query usage events once]
    E --> F[Collect activity events and unlock timestamps]
    F --> G[Parse sessions and cache app labels for this call]
    G --> H[Save sessions and unlocks]

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_{Reviews (2) · Last reviewed commit: "perf(watcher): cut redundant work from e..." · Reviewed by Greptile}

[0xbrayo]

🤖 Claude, on behalf of @0xbrayo

@greptile review