Load odm manifest.xml and fallback to vendor.

Eventually, we may pursue properly overlayed manifests.
However, adding this fallback mechanism so that the ODM image
can specify a more specific image than the vendor image can.

Bug: 65032103
Test: vendor manifest still loaded properly
Test: vintf_object_test + libvintf_test
Change-Id: Iccb1f5937755f78c9e50565aeb19e0a580e7e577
2 files changed