Fix setApplicationNightMode crash due to permission denial.

Test activity crash when calling UiModeManager#setApplicationNightMode.
There should call clearCallingIdentity before updating configuration in
system server.
Also clear some code around those methods.

Fixes: 189166288
Test: atest SplashscreenTests#testSetApplicationNightMode
Change-Id: I163919927030f81739aa3d9520e2012a92028793