SurfaceFlinger: add a test for unknown offset

Add a unit test that tries to get phase offset for a
refresh rate that is not part of the boot list.
The scenarios simulates an external display hotplug
with different refresh rate than the internal one.

Bug: 151859083
Test: adb shell /data/nativetest64/libsurfaceflinger_unittest/libsurfaceflinger_unittest
Change-Id: I3adee92b91d0c1a337c9a6ffd7431e7b854bbe0d