Support build targets with no installable apps/packages

Specifically, do not error out if no installable apps are found.

- When creating an archive of all the apps, and if no apps exist
for the target, generate an empty archive file.
- If building for a target with no installable jar(s) or apk(s),
generate an empty package stats file.

The former is just an output artifact. The latter is used
by the upload_pkg_stats.py script which correctly handles
an empty input file.

BUG: 23421592

Change-Id: I48db9f9e1f61914d8fd938130e09b41849685450