Support to build dpi-specific apk variants.

In unbundled apps_only build, in addition to the base apk, you can also
build the dpi-specific apk variants, with:
LOCAL_DPI_VARIANTS := <a list of dpi names>

Previously user needs to include $(BUILD_PACKAGE) repeatedly with the
same package definition except dpi flags.
With this change, all the dpi-specific apk variants share the base apk's
compiled Java code and only diverge at the point we add resources/assets
to the apk.
Also we set up variables/targets/rules in a way those dpi-specific apks
appear to be independent apks to the users, for example, you can pass
"AppName_<dpi_name>" to tapas, and AppName_<dpi_name>.apk lives in its
own intermediate directory.

Bug: 18388705
Change-Id: I2ba4972ea7d1f796352fab2407888f996781ae44