Add LOCAL_MODULE_HOST_OS

Instead of wrapping a host module definition in 'ifeq($(HOST_OS),...)'
in the Android.mk files, define which hosts are supported using
LOCAL_MODULE_HOST_OS.

A blank LOCAL_MODULE_HOST_OS means that linux and darwin are supported.
A non-empty LOCAL_MODULE_HOST_OS lists the supported HOST_OSs.

Change-Id: I1e342d1908cfa00aef2c39c145b4f5f81c373bc6