Fix PermissionHelper NullPointerException

During the FOTA process, the app can be deleted depending on the
situation.
If the app is deleted, PackageInfo needs a null check because it can be
returned null.

Test: FOTA from S OS to T OS
Test: System app is deleted by policy during FOTA process
Test: Monitor notification permission migration.
Change-Id: I72a69c3e8998f3182cec10dd1583020096204ad4