fastboot: get rid of manual transport memory management

Existing code has transport memory leaks. Use smart pointers
for transport to get rid of those cases and manual memory
management

Test: atest fastboot_test
Test: manually checked transport isn't leaking anymore
Bug: 296629925
Change-Id: Ifdf162d5084f61ae5c1d2b56a897464af58100da
Signed-off-by: Dmitrii Merkurev <dimorinny@google.com>