Make the default getTimeSpentInApp() return empty

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