SF: fix frame rate for layer tree

With the current implementation we might mark layers as NoVote
incorrectly when a sibling has a frame rate.

Test: adb shell /data/nativetest64/libsurfaceflinger_unittest/libsurfaceflinger_unittest
Bug: 195205467
Change-Id: I62641f855a027e1192f9a85a4bc50692b1744764