Refactor ZygoteProcess to deal in LocalSocketAddress.

Currently ZygoteProcess only uses String names in the RESERVED socket
namespace. This CL reworks the class to use LocalSocketAddress, so that
other socket namespaces can be used to communicate with zygotes.

Bug: 63749735
Test: m (no functional change)
Change-Id: I4146f684bfcd78b16500829d02ff54590a8b48f5