It's moved from PrepareStreamingService intent service.
Now PrepareUpdateService takes an UpdateConfig and
builds PayloadSpec for UpdateEngine for both streaming
and non-streaming update.
It allows us to do all preparations in intent service's
thread, without blocking UI.
We will also add checksum verification to
PrepareUpdateService.
Test: device, junit
Bug: 77150191
Change-Id: Iea69acd9aa41e17538c26aff60f7598093ca7744
