SF: Remove display config functions from HWC2

HWC2 is unnecessary stateful stopgap between HWComposer and
ComposerHal. In this CL the following functions are removed from HWC2
  * getActiveConfig
  * getActiveConfigIndex
  * getDisplayVsyncPeriod

and ComposerHal is called directly from HWComposer. This way display
configs are stored only in HWComposer.

Additionally HWC2::Display::Config is renamed to DisplayMode and
it's extracted in its own file.

From the perspective of SurfaceFlinger this CL is not modifying
behaviour.

Bug: 159590486
Bug: 175678215
Test: atest libsurfaceflinger_unittest
Change-Id: I8cb450209adf038d891cff00d1c2690c8e6d94f7