Add PrepareUpdateService.

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: I15c0bc58e3238bea6ea1c4f13063575e2def89c1
Merged-In: Iea69acd9aa41e17538c26aff60f7598093ca7744