SurfaceFlinger: expose vsync id extra buffers

Expose a function for clients to call and query the number
of extra buffers needed when vsync id is passed with the buffer.

Test: launch an app and observe systrace
Test: SF unit tests
Bug: 178148035
Change-Id: Icbeec66073feeae9768f0dcc45831b26144ab6f6
10 files changed