VPLAY-13176 underflow detection refactoring

[pstroffolino]

Reason for Change: motivated by struggles getting L2 6002 to pass on OSX
Refactoring to eliminate polling loop, vendor specific code, dependencies on streamer-level signaling.

Test Guidance: underflow monitoring/rerporting must work well across devices and simulator variations. All underflow-specific tests must work.

Risk: High - simplification, but big change. That said, current implementation has some known issues.

[Vinish100]

Looks good to me 👍
L1 is failing as fakes are not updated

[Copilot]

Pull request overview

Refactors video underflow detection to a timer/deadline-driven monitor, removing steady-state polling and introducing explicit notifications from fragment download and buffering transitions to improve consistency across devices/simulators.

Changes:

• Replace polling-based underflow detection with a deadline-based AampUnderflowMonitor that arms/disarms on fragment and buffering notifications.
• Wire underflow-monitor notifications into fragment download/chunk caching and buffering state transitions.
• Update unit-test fakes to match the new monitor API and stream-abstraction notification surface.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
AampUnderflowMonitor.h	Redefines the underflow monitor as a timer/deadline-driven component with notify-style APIs.
AampUnderflowMonitor.cpp	Implements deadline arming/disarming, deadline wait loop, and underflow declare/resume behavior.
StreamAbstractionAAMP.h	Adds stream-level notification methods for underflow monitor events (fragment queued, pipeline paused/resumed).
streamabstraction.cpp	Updates underflow monitor construction and forwards new notifications to the monitor with locking.
priv_aamp.cpp	Notifies the underflow monitor when buffering pauses the pipeline; documents why resume notify isn’t called.
MediaStreamContext.cpp	Notifies the underflow monitor on successful video fragment/chunk availability; adds a race-avoidance guard for TSB discard logic.
test/utests/fakes/FakeAampUnderflowMonitor.cpp	Updates fake monitor constructor and stubs new notify APIs for unit tests.
test/utests/fakes/FakeStreamAbstractionAamp.cpp	Adds stubs for new stream-level underflow monitor notification methods.