[Security] resolve overlay attack on RequestIgnoreBatteryOptimizations

we should put the addSystemFlags() after onCreate() method rather than the original onStart() method (refer the proposed solution in b/199167909)

Bug: 199167909
Test: make SettingsRoboTests
Change-Id: Ic7361443bfa2e1a6d8d962708c794fa0122a691c
1 file changed