libcutils: Make uevent_kernel_* namespace-aware

This change adds user namespace-awareness to uevent_kernel_* in
libcutils. Instead of assuming that root is always uid 0, it detects
whether the uid 0 is mapped in the current user namespace and returns
the appropriately mapped uid (or the kernel's "overflowuid" in case it
is not mapped).

In older kernels, or those where user namespaces are not enabled, this
still uses uid 0 for root.

Bug: 62378620
Test: bullhead networking still works
Test: Android in Chrome OS can now receive netlink-related messages

Change-Id: I7ea3454e8f38b9c70c65294d6b2a99e5a88f9d70