Make setActiveConfig() private, remove plumbing

This is to address leftover code review feedback from ag/9714241. We no
longer use setActiveConfig() except for a call from an internal debug
command, so make it private, and remove the plumbing to support calling
it from other processes.

Bug: 142507213
Test: atest libsurfaceflinger_unittest
Test: atest libgui_test
Test: atest SurfaceFlinger_test
Test: atest sffakehwc_test
Change-Id: I8ae9c2636fc009a7ebeba06a72913a37a6af8ef4