Background distGzipFile to speed up CI builds

These can take a minute or more of build time, and currently hold up
other processing. We could move to a parallel compression scheme to
speed them up, but it's simpler just to background them and continue the
build, waiting to make sure they've finished before exiting.

Testing this on git_master/flame-userdebug:

This brings a local no-op `m nothing dist` build from 2m1s to 1m39s,
even though most of that time is still spent waiting on these (since
there is very little else happening in a `nothing` build when Soong and
Kati don't need to run).

Running `touch art/Android.mk; m nothing dist` (so the above, but with
Kati) goes from 3m39s to 2m2s. We spent 9 seconds at the end waiting for
these to finish.

CI cases that almost always run Kati plus some number of other actions
are likely to completely hide the time taken to dist these files.

Bug: 229932999
Test: Check $DIST_DIR, files still exist
Change-Id: I1fb78d7c4d5103d72b5d71d9277dea0d452f8968