Use the prefix in product_id for system_id.

If product id is "android-things-test:123", but system id is
"android-things:som-rpi3", we change the system id to
"android-things-test:som-rpi3".

This is needed for devconsole testing to isolate test environment
with production.

Also added unittest for image_properties_android.cc

Bug: 62466250
Test: check system_id in Omaha request
Test: update_engine_unittests

Change-Id: Ib97573e117c0fecf85922260e6e75e63e36c18e3
(cherry picked from commit 98c0751f0dc970cadfc8014883018524dd8899f1)