Libcore: Rename Posix to Linux

We refer in Os to Linux manpages. Future changes will add Linux-specific
APIs to Os. Be honest and rename the class.

Bug: 36232535
Test: m
Test: m test-art-host
Test: Device boots
Change-Id: Ie9d4536d03499a75efaf2bcf16d41bacc97ac943
6 files changed