add sideload-host mode to adb

The sideload-host mode turns the host into a server capable of sending
the device various pieces of the file on request, rather than
downloading it all in one transfer.  It's used to support sideloading
OTA packages to devices without the need for them to hold the whole
package in RAM.

If the connected device doesn't support sideload-host mode, we fall
back to the older sideload connection.

Change-Id: I5adaedd8243dc3b76414bba0149879ca2bbf35fa