Remove fastboot state management

Using async, there is no need to separately track fastboot command and
download state. The download code can just fold into the download
command function and execute asynchronously.

The CL simply removes the struct Fastboot and makes all methods regular
helper functions. Logic for handling the download state is moved into
download command helper function.

Bug: 353982966
Change-Id: Iaad15ba2f7f6b2b2c9da37806778c4cd5cb93edc