Create new conditional userdebug_or_eng

Create a new m4 macro called userdebug_or_eng. Arguments
passed to this macro are only emitted if we're performing
a userdebug or eng build.

Merge shell.te and shell_user.te and eliminate duplicate
lines. Same for su.te and su_user.te

Change-Id: I8fbabca65ec392aeafd5b90cef57b5066033fad0