Apply the light theme for PerfTestActivity

This CL restores the window background color back to white. The IME
color is  light as well. So when the IME is showing and hiding, the
gesture navigation bar won't need to change its color to adapt the
background, which might consume additional CPU resource.

Fix: 334024581
Flag: NA
Test: ImePerfTest#testShowImeWarm
Change-Id: I83bf308933d63cb138353d682a66cb8f6e1eb07a