Introduce a default density ('mdpi') for all device builds.

This is done by appending "mdpi" (medium density or 160 dpi, which is what
we've been using so far on dream etc.) to PRODUCT_LOCALES if no other dpi is
found there. (PRODUCT_LOCALES is then sent as a configuration parameter to
aapt to determine which resources should be included in the build.)

With this change, each build will now include only one density of any given
asset. For example, if there exists

  drawable-hdpi/foo.png
  drawable-mdpi/foo.png

...then the build will include *only* drawable-mdpi/foo.png, unless the
density is overridden in the build's own device spec:

  PRODUCT_LOCALES += hdpi

...in which case the build will include only drawable-hdpi/foo.png.