Refactor nullchecks

Move nullchecked earlier in code flows, add nonnull annotations where
necessary.
Refactor notification sensitive check into boolean method that doesn't
repackage as PackageInfo

Bug: 331946904
Test: atest SensitiveContentProtectionManagerServiceNotificationTest
Flag: EXEMPT - strict refactor and null checks
Change-Id: Ib100da3a01010019685d6806f92a8fe04ccbfab0