fastbootd: Fix transport ownership.

This change moves Transport ownership back out of FastBootDriver.
Callers of set_transport must ensure that the previous transport is
destroyed. In addition, deleting a transport now ensures that it is
closed.

Bug: 78793464
Test: fastboot, fuzzy_fastboot works
Change-Id: I8f9ed2f7d5b09fd0820b2677d087a027378f26db