(This is a merge of http://go/wvgerrit/70303)
This adds a platform.h file to abstract some of the differences
between Windows and POSIX platforms. This includes ntohl, setenv,
and ssize_t.
Bug: 122953649
Test: Android Unit Tests
Change-Id: I3235f3f284b53d24d7365ff3f4a06dcd9b403697