AOD: Fix crash when AOD is stopped immediately after starting

Fixes an issue where the screen state was set after DozeService was already
destroyed, causing a crash.

Change-Id: I13ff590b62f905330ccb438692410e118b76c242
Fixes: 64907936
Test: runtest -x $ANDROID_BUILD_TOP/frameworks/base/packages/SystemUI/src/com/android/systemui/doze/DozeScreenState.java