Runtime resource overlay: clean-up.

This is a combination of three minor clean-up tasks:

- Generate correct idmap even if name lookup fails:

  Any resources following a resource with a spec but no actual values
  would receive an incorrect ID in the idmap due to an accumulating offset
  error. To combat this, a dummy value is inserted in the idmap whenever
  a resource with a spec but no values is encountered.

- Optimize pruning of padding zeroes in idmaps:

  Earlier, trailing zeroes were iteratively popped from the end of each
  type block of an idmap. This commit instead tracks where to make the
  cut, and does so with a single function call.

- aapt: warn about resources which lack values:

  The resource framework assumes every resource to have a value for
  at least one valid configuration. However, if multiple resource
  directories are used (several -S options to aapt), the list of
  configurations is limited by dpi (-c option to aapt) and a resource
  is only available in discarded dpi configurations, aapt will create
  a resource entry where each configuration lack an actual value. This
  commit lets aapt emit a warning whenever this has happened.

Change-Id: Ic7451b7f4adfef5bfa6b0d7e64e057f317a2cdaa