Rename getNotificationChannelsBypassingDnd argument to uid

It said "userId" in NotificationManagerService and PreferencesHelper, but in practice was expecting its input to be a uid based on how it's implemented. It also seems most users were already passing in uids (particularly NotificationBackend).

Bug: 220469720
Test: manual verification of settings page, NMSTest, PreferencesHelperTest
Change-Id: I432bdfc7d2986a0a1111c210fd89671524b3885c