fw_update: target specific startUpdate implementation

Description:
This commit implements the `startUpdate` handling of the target specific
update flow [1], which introduced `ItemUpdateManager` for handling
target specific updates.

Motivation:
We have introduced the `FirmwareInventory`/`FirmwareInventoryManager`
classes to manage D-Bus interfaces for firmware update, After that, we
would like to implement the `StartUpdate` method for `FirmwareInventory`
properly for the update process implementation, which
`AggregateUpdateManager` will help route the command to the specific
update sessions.

Test results:
- Successful FD item Firmware update performed on Yosemite4, by manually
  triggering the `StartUpdate` method call on the
  `xyz.openbmc_project.Software.Update` interface.
- Successful FD item Firmware update performed on Yosemite4, by using
  Redfish API to trigger the update.
  (main stream version, no patch needed)
- Successful firmware update performed on Yosemite4 via the dir inotify
  based flow.
- Successful firmware update performed on Yosemite4 via the multipart
  image flow.
- Successful firmware update performed on Yosemite4 after a previous
  failed update attempt.
-

[1]: https://github.com/openbmc/docs/blob/master/designs/code-update.md

Change-Id: I724f0987eb882a1b4e6bce8c209b319c927689e9
Signed-off-by: Unive Tien <unive.tien.wiwynn@gmail.com>