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)
Merged-In: I4146f684bfcd78b16500829d02ff54590a8b48f5
Change-Id: I0113a7189530c2e57c48058df542057e855bae42