Add verification before downloading whole package

UpdateEngine has a feature that verifies
payload without downloading the whole update package.

If UpdateEngine detects invalid payload,
the sample app aborts the update.

No JUnit tests, because it accesses files on the
device and migrating tests to robolectric
is not worth for this sample app.

Bug: 77150191
Test: device
Change-Id: Ib8ce73508a02cf5fdcb326d8ba46c1d05ed5efe5