libgui: add a test for Sensor serialization.

On a few test boards we've hit a problem with Sensor serialization
causing a SIGBUS due to misaligned access. This is being fixed in
Flattenable.h, but adding a test here to verify the fix and detect
regression seems like a good idea.

Bug: http://b/31671510
Test: adb shell /data/nativetest/libgui_test/libgui_test \
                --gtest_filter=SensorTest.*

Change-Id: I233359b04789dabe2246b95051290e101e0bd2c1