Add BUILD_USERNAME and BUILD_HOSTNAME

As part of a future change to sandbox the build on Linux, the real
username will be switching to "nobody", and the hostname will be
switching to "android-build".

The USER environment variable will reflect the sandboxed value, so for
the build properties that want the external USER, they'll need to use
BUILD_USERNAME.

Similarly, BUILD_HOSTNAME will reflect the real value, while the
`hostname` tool will return "android-build"

Bug: 122270019
Test: check build.prop
Change-Id: I99604b9488732a63690b256dc4dd7894d369a32c