Update statementservice assetlink.json parsing

This updates the statement service json parser to parse the new
relation_extensions introduced to the digital assetlinks protocol for
dynamic app links.

The parser is also changed to take the last value in a json object if
multiple values for the same key is found. This is done to make it
consistent with the json parser used in the DAL service used by GMS Core
devices.

Bug: 307557449
Test: manual
Flag: EXEMPT external library
Change-Id: Id550a53f2aa932959f27ecdb9556b6dde0c5fb52