SF: Mutable RefreshRateConfigs

Modify the list of supported refresh rates in RefreshRateConfigs
on hotplug. In order to support this the RefreshRateConfigs class
is refactored to not return references to its internal members,
but instead return copies. This won't be expensive because
the RefreshRate objects which are returned are small.

Bug: 159590486
Test: 1. boot w/o display
      2. connect a display which supports multiple refresh rates
      3. request a mode switch from an app
      4. verify that the switch works
Test: atest libsurfaceflinger_unittest
Test: toggle refresh rate overlay
      adb shell service call SurfaceFlinger 1034 i32 1/0
Change-Id: I1588bf004a0a0319a94931adbfb822836703849a