The "Open by Default" in App Info page.
Bug: 236346018
Test: Manual with App Info page
Test: Settings Unit tests
Change-Id: I20f827241ff46bca28440b56fd32a0712ee439f9
Current ApplicationFeatureProvider.getTimeSpentInApp() defaults returns
null, in extreme case there is ACTION_APP_USAGE_SETTINGS handler on
device but getTimeSpentInApp() still returns null, may lead
AppTimeSpentPreference to crash.
Make it returns empty string instead to eliminate this extreme case.
Bug: 236346018
Test: Unit test & Manual with SpaActivity
Change-Id: I33b413ceb9bdf48e3902e02c91fab617fe297862