[SurfaceFlinger] fix permanently enabling early offsets.

* Add notion of intended period in DispSync, and use that to detect
whether the simulated period will be changed.
* Propagate that signal to setDesiredActiveConfig, so that we don't fall
into early offsets unnecessarily, which can cause early offsets to
always be enabled.

Bug: 132678707
Test: systrace
Test: swappy test app
Test: scrolling through google news
Change-Id: I18df1b9d949cd534ecbf1c8891b6f88eab8be399
Merged-In: I18df1b9d949cd534ecbf1c8891b6f88eab8be399