Build update_engine_sideload.

Add a new "sideload" executable target that applies an update payload
directly using the UpdateAttempterAndroid.

This initial CL buils a dynamically linked program that's targeted to
run in the system image for now, but will later be transformed into a
static binary to run from the recovery environment.

Bug: 27178350
TEST=Applied a payload directly on a device using:
`update_engine_sideload --payload=file://foo/bar ...`

Change-Id: I289a724d013abdc390187d669dccd3edf2fd3434