Parse multiple <app> tag.

SoM and product will have different app id in Android Things, and in
Omaha response they will be in separate <app> tag.
Each <app> will have its own <urls>, <packages>, <actiions>, etc.

Bug: 36252799
Test: update_engine_unittest
Change-Id: I47f46155a7362dba2a3010b4372a8f254c78fb30
(cherry picked from commit 558084b960a1b0a81e43395c12400c54a4b0c3c0)