Add flag for native session manager

Adds a flag for the upcoming "SessionManager" in PowerAdvisor, which
holds the logic for the Timeline API and HAL communication. This flag
guards the creation of SessionManager, and by extension all of the
associated Timeline API logic.

Bug: 367803904
Change-Id: I76200fa5bf4f3f519ec20a8172e45b7848ab124c
Flag: com.android.graphics.surfaceflinger.flags.adpf_native_session_manager
Test: presub