RUM-17613 Flush the pending timeseries batch on backgrounding

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What does this PR do?

Makes the timeseries collector flush its pending batch when the app leaves the foreground,
instead of dropping whatever was buffered.

The four-state AtomicReference<State> enum is replaced by a generation-counted State
guarded by synchronized. Leaving the foreground schedules a deferred suspension
(SUSPEND_DELAY_MS = 200) which, if the app is still not in the foreground when it fires,
deactivates the current generation and flushes every pipeline. Returning to the foreground
cancels a suspension that has not fired yet, or starts a fresh sampling generation if it already
did. Sampling now only runs on a foreground view, so onSessionStart needs to know which view
the session begins on — TimeseriesCollector.Factory.create takes a viewType, supplied by
RumSessionScope from the active RUM context.

Because sampling is unconditionally suspended outside the foreground,
TimeseriesConfiguration.Builder.collectInBackground no longer has anything to switch and is
removed together with its tests.

Motivation

Without this, a batch that is still filling when the user backgrounds the app is silently lost:
sampling was simply paused, and the buffer was only drained on session stop. The 200 ms debounce
mirrors ActivityViewTrackingStrategy.STOP_VIEW_DELAY_MS, which guards the same race — an
Activity-to-Activity transition briefly leaves no active view when the tracking strategy stops
the view on pause rather than on stop, and that must not be mistaken for backgrounding.

Additional Notes

The new State has no terminal STOPPED, so onSessionStart after onSessionStop would
restart sampling. That invariant is now the caller's: RumSessionScope.stopTimeseries() swaps in
a NoOpTimeseriesCollector, and a fresh collector is built per session. The test that asserted
the old terminal behaviour is dropped for that reason.

The three AtomicReference<DefaultTimeseriesCollector.State> detekt safe-call entries go away
with the AtomicReference.

api/apiSurface and api/dd-sdk-android-rum.api are unchanged — every removed member was
internal (verified by re-running generateApiSurface + apiDump).

Instrumented tests are intentionally not part of this PR; they arrive with the last PR of the
stack.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

Ref: RUM-17613

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Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• RUM-17613 Add integration tests for timeseries collection #3713
• RUM-17613: timeseries [2/5] Using updated timeseries schema from rum_event_schema #3712
• RUM-17613 Flush the pending timeseries batch on backgrounding #3715 👈 (View in Graphite)
• RUM-17613 Rename Timeseries abstractions to TimeseriesCollector #3711
• feature/timeseries

This stack of pull requests is managed by Graphite. Learn more about stacking.

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⚠️ Warnings

🚦 8 Pipeline jobs failed

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ℹ️ Info

No other issues found (see more)

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 100.00% (+27.18%)

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Superseded: the background-collection fix now sits after the schema PR in the stack and carries the full RumContext migration. Replaced by #3717 (branch renamed to tvaleev/feature/RUM-17613-3-background-flush).