perf: Schedule session end on shared executor (JAVA-653)

[runningcode]

📜 Description

LifecycleWatcher created a java.util.Timer (a dedicated thread) on the first app-background transition, and that thread lived for the rest of the process. The session-end task now runs on the shared timer executor (SentryOptions#getTimerExecutorService), already used for transaction timeouts, whose single worker thread is reused and self-terminates when idle.

The task is scheduled/cancelled as the app moves between background and foreground via Future#cancel(false) (the same pattern SentryTracer uses for idle/deadline timeouts). If scheduling fails (executor already shut down), the session is ended immediately instead of being left open.

💡 Motivation and Context

Part of reducing the number of threads created by the SDK: JAVA-653.

On Android this timer thread was created on the first background transition and never went away. Reusing the shared timer executor removes it.

💚 How did you test it?

Existing LifecycleWatcherTest (updated to assert on the scheduled Future instead of the removed Timer/TimerTask internals) and SessionTrackingIntegrationTest. Both fixtures now wire a real executor into options so scheduled tasks run.

📝 Checklist

• I added GH Issue ID & Linear ID
• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• I updated the wizard if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

🔮 Next steps

Related PRs in this effort: RateLimiter (#5814), performance collector (#5816), HostnameCache (#5817), batch processors (#5818).

🤖 Generated with Claude Code

[linear-code]

JAVA-653

[sentry]

📲 Install Builds

Android

🔗 App Name	App ID	Version	Configuration
SDK Size	io.sentry.tests.size	8.49.0 (1)	release

⚙️ sentry-android Build Distribution Settings

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	322.13 ms	363.14 ms	41.01 ms
Size	0 B	0 B	0 B

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
eb95ded	317.51 ms	369.08 ms	51.57 ms
2124a46	319.19 ms	415.04 ms	95.85 ms
c3ee041	310.64 ms	361.90 ms	51.26 ms
dcc6bbf	382.58 ms	462.13 ms	79.54 ms
7a19fee	315.46 ms	368.62 ms	53.16 ms
9054d65	330.94 ms	403.24 ms	72.30 ms
d500866	326.13 ms	378.70 ms	52.58 ms
ad8da22	365.86 ms	427.00 ms	61.14 ms
ed33deb	337.52 ms	484.06 ms	146.54 ms
d15471f	310.26 ms	377.04 ms	66.78 ms

App size

Revision	Plain	With Sentry	Diff
eb95ded	0 B	0 B	0 B
2124a46	1.58 MiB	2.12 MiB	551.51 KiB
c3ee041	0 B	0 B	0 B
dcc6bbf	1.58 MiB	2.12 MiB	553.10 KiB
7a19fee	0 B	0 B	0 B
9054d65	1.58 MiB	2.29 MiB	723.38 KiB
d500866	0 B	0 B	0 B
ad8da22	1.58 MiB	2.29 MiB	719.83 KiB
ed33deb	1.58 MiB	2.13 MiB	559.52 KiB
d15471f	1.58 MiB	2.13 MiB	559.54 KiB