Improve some External Storage Notification texts on TVs

Customised strings (only for TV form factor):
- ext_media_new_notification_message
- ext_media_unsupported_notification_message
- ext_media_ready_notification_message
- ext_media_checking_notification_message

Test: N/A - only string changes
Bug: 181638310

Change-Id: I63152ca960af2639c72e197e149459a2aa1cac1a