Update some Nullable annotations on PendingIntent

It's possible to statically verify that mTarget can never be null, and
that in every instance where an @NonNull annotation has been added here
it was previously impossible for null to be returned at that location.
The only exception is for getCreatorPackage(), where it is more
difficult to verify that it is impossible for this to be null.
Logically, it doesn't make any sense for a PI to not have a creator
package, and I have gone through every call flow for setting the creator
package I could fine to verify it can never be set to null.

Bug: 175221460
Test: manual sanity checks
Change-Id: Ia9e2c3c4ff8c9082190cd50129cedcbcfd219574