New command line tool 'idmap'

Introduce a new tool 'idmap' to handle generation and verification of
idmap files. The tool is modelled on 'dexopt', and is intended to be
used similarly, notably by 'installd'.
See cmds/idmap/idmap.cpp for further documentation on 'idmap'.

Note: this commit is interdependent on a commit in project build/ to add
'idmap' to PRODUCT_PACKAGES.

Note: the changes to androidfw are only stubs. The actual implementation
will be provided in Runtime resource overlay, iteration 2.