fastbootd: pass handle in place of fd

There are cases such as non 4KB aligned buffers which break writes on
file descriptors opened with O_DIRECT. Flashing functions need more
control over the file descriptor, which is not achievable easily by
having only a file descriptor passed. This CL modifies the logic to pass
the PartitionHandle in place of the file descriptor during flashing.

Bug: 225108941
Signed-off-by: Konstantin Vyshetsky <vkon@google.com>
Change-Id: I049d0ffb183c9de1267b7e442488c8ba5002186a