Use PersistableBundle.getInt(String, int) with default value parameter

The default value for PersistableBundle.getInt(String) is 0. This
value is a valid value for the KEY_PREFERRED_IKE_PROTOCOL_INT
setting which is PREFERRED_IKE_PROTOCOL_AUTO. But the preferred
design now is to use IPv4 instead of AUTO. The default value for
getting KEY_PREFERRED_IKE_PROTOCOL_INT should be explicitly set
to an invalid value so that the flow will go into the default
case as expectation.

Bug: 269714970
Bug: 284246686
Test: m ; flash and check the value
Change-Id: Icf83ec1a50972b2d9ce7d168dad75dd9aa3cf1bb