SF FlagManager: Add a method for checking multithreaded_present

Factored out of Ib9d074671e32c95875ef7e0791dd95d6e595e47a to avoid
more potential rebases. That CL will use the flag.

Bug: 241285491
Bug: 259132483
Test: adb shell dumpsys SurfaceFlinger | grep multithreaded_present
  -- verify flag shows up default false.

Change-Id: Id3ba46cbc0c9487bf3b29e7b9d4de58e5ee8e459