Only dist apex sizes for checkbuild

Using $(call dist-for-goals,droidcore,...) for every apex
installed-files.txt causes every apex to be built for all builds,
causing a signficiant regression in build time.  For now only
dist them in checkbuilds, which were going to build all of them
anyways.

Ideally we would like to dist the installed-files.txt only if
the apex was already built, but there is not currently a way to
express that.

Test: treehugger
Bug: 149979076
Change-Id: I21edbc90980f94ad6d497cb86ee80223dd782fe1