Bug 887121 - Make packager install and szip .so libraries in assets/ directly. r=glandium

This adds a Component type to the mozbuild.mozpack package manifest
parser, and teaches the packager to accept components of the form
[name destdir="dir"].  Then we update the Android package manifest and
simplify the packager code.

I would have liked to make the packager put mozglue.so and
MOZ_CHILD_PROCESS_NAME in lib/$(ABI_DIR) directly, but this turned out
to be awkward.  Since MOZ_CHILD_PROCESS_NAME needs to have lib/ in its
name to load successfully on Android, we would have to add notation in
package manifests to install bin/lib/*plugin-container* to
lib/$(ABI_DIR)/*plugin-container*.