Fix crash in ExternalSourceDetailPreferenceController.

- Add null check for the package info when trying to retrieve the
install app state. During package update, the package info might becomes
null. The App info activity will finish in that case, but this method
might be called before the activity actually finishes.

Change-Id: I18ff448666dfb403bb6693fccd3e54276db36f37
Fixes: 78257414
Test: make RunSettingsRoboTests