OsConstants: Add SOCK_CLOEXEC and SOCK_NONBLOCK

Android makes the function socketpair() publicly visible, but never made
SOCK_CLOEXEC nor SOCK_NONBLOCK visible. Java socketpair() users have no
approved flag to ensure sockets are immediately created with O_CLOEXEC,
nor set the non-blocking status on sockets.

socketpair() documentation:
https://developer.android.com/reference/android/system/Os.html#socketpair(int,%20int,%20int,%20java.io.FileDescriptor,%20java.io.FileDescriptor)

Test: compiles
Test: cts-tradefed run cts -m CtsLibcoreTestCases -t libcore.android.system.OsConstantsTest
Bug: 120983106
Change-Id: Iba6bd318b14faad1696e5ea5d346d2f2f85b8044