Get OperationType from transport

This CL was merged earlier (ag/13484966) and then reverted due to the
new behaviour breaking D2D transfers.

Merge it again with all changes being controlled by a flag (default
off), see UserBackupManagerService:getOperationTypeFromTransport in this
CL. View the diff between patchsets 1 and 2 to only see what's changed
between the earlier reverted code and the fixed version of it (i.e. with
the flag).

The flag can be changed via adb for now, we will set it to true by
default once other components are ready.

Bug: 174216309
Test: atest UserBackupManagerServiceTest
Change-Id: I7473c9b4f8d0c4d20155be76930279184ffb17c4