NotificationManagerServiceTest: Mark unaudited PendingIntents with FLAG_MUTABLE_UNAUDITED

These PIs need an explicit mutability flag. This temporay flag is being
used to mark these points in code and should be replaced ASAP.
See go/immutable-pendingintents for more context.

Bug: 160794467
Test: TH
Exempt-From-Owner-Approval: noop change
Change-Id: Ice453974aafd943722c0ad44e017fdf1ca3237a9