Reproduce RUMS-5469: TTID not emitted when launch Activity finishes before first draw

[mariusc83]

Summary

Reproduces RUMS-5469: Android TTID not emitted on emulators (API 34) when an interstitial
Activity (e.g. AuthenticationActivity) calls finish() before its decor view completes a
single draw pass.

• Test 1 (RumFirstDrawTimeReporterTest): Asserts that when the decor view detaches from
  the window before onViewAttachedToWindow fires, onFirstFrameDrawn is still called with
  a fallback timestamp. FAILS in buggy code because onViewDetachedFromWindow is an empty
  body with no fallback.

• Test 2 (RumAppStartupDetectorImplTest): Asserts that when a second Activity is created
  while the first is still alive (interstitial pattern), onAppStartupDetected is called for
  both Activities so that TTID can be tracked for the one that actually draws. FAILS in buggy
  code because onBeforeActivityCreated only fires when numberOfActivities == 1.

• Test 3 (RumSessionScopeStartupManagerTest): Asserts that when reportAppFullyDisplayed
  is called and TTID was never received (because the first Activity's view detached before
  drawing), a TTFD event is still written. FAILS in buggy code because onTTFDEvent returns
  early with a warning and never writes anything when TTID has not yet been reported.

Test plan

• Run ./gradlew :features:dd-sdk-android-rum:test — all three new tests should FAIL
  against the current code, confirming the bug exists
• Verify no existing tests are broken by the new test additions

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