fastbootd: add support to reset fd by handle

There are cases where the flags the file descriptor was opened with need
to be modified. This CL adds functionality to reset the file descriptor
held by a PartitionHandle, reopening with new flags and repositioning
file offset to previous.

Bug: 225108941
Signed-off-by: Konstantin Vyshetsky <vkon@google.com>
Change-Id: I9adb0e7696bc6af74e14dd61a6cb0ef10b4c98c8