[Media TTT] Add the #transferToReceiverTriggered callback.

Also renames TransferInitiated -> TransferToReceiverTriggered, because
we also need a TransferToThisDeviceTriggered callback.

Bug: 203800643
Bug: 203800347
Test: verify `adb shell cmd statusbar media-ttt-chip-add-sender Tablet
TransferToReceiverTriggered` works
Test: media.taptotransfer tests

Change-Id: Ibb968d5c35391cf84e28fc2460148ea6bf396d14