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

apex/androidmk.go

@@ -308,7 +308,7 @@ func (a *apexBundle) androidMkForType() android.AndroidMkData {
 				}
 
 				if a.installedFilesFile != nil {
-					goal := "droidcore"
+					goal := "checkbuild"
 					distFile := name + "-installed-files.txt"
 					fmt.Fprintln(w, ".PHONY:", goal)
 					fmt.Fprintf(w, "$(call dist-for-goals,%s,%s:%s)\n",