SurfaceFlinger: fix setDesiredActiveConfig

setDesiredActiveConfig is trying to initiate a config change
even if the desired config is alreadt the active config. This creates
an issue where mVSyncModulator->onRefreshRateChangeInitiated() is called
but the counter part mVSyncModulator->onRefreshRateChangeCompleted()
is never called since the refresh rate never changed.

Test: collected systrace on taimen
Bug: 150647346
Change-Id: I2659183bf2a4e1676627d84379f4d533bdb9b5fc