SurfaceFlinger: add more cleanup to unittests

Cleanup the display vectors from Current and Drawing state
of TestableSurfaceFlinger to make sure we don't have any mock
leaked objects.

Bug: 127332154
Test: adb shell /data/nativetest64/libsurfaceflinger_unittest/libsurfaceflinger_unittest
Change-Id: I58f3339ad6ad7b01da6861e07b759611dbd2f9cc