Add strings to use in Bluetooth device forget confirmation

We're going to add a confirmation dialog for when the user chooses to
unpair with (or "forget") a Bluetooth device on the device details
page. These are the strings we'll use in the dialog.

Bug: 37955181
Bug: 35876580
Test: (should be covered by existing tests)
Change-Id: Iec08fcc2730129c0e9a8f54cc94ab8aad958f353