Support to override instr target package in inherit-package

http://b/issue?id=2451361
With this CL, you can inherit a testing package to test a inheriting package.
Note: we change the meaning of $(5), so we need to change any existing calls
to inherit-package in other projects.

Change-Id: I673155d1b8b6006953d14e1ab87066536ebd1e54