Stop using DIST_DIR directly, use dist-for-goals instead

As a preparation in refactoring how we handle dist, I'm making
everything use dist-for-goals. In this case, instead of fully relying on
a phony rule, write the gtest xml into an intermediate folder, and use
dist-for-goals to copy that into the dist dir.

Bug: 117463001
Test: DIST_DIR=dist m aapt2_run_host_unit_tests dist
Test: DIST_DIR=dist m aapt2_run_host_unit_tests dist   (reruns test)
Change-Id: I166c2b3088703875c6028c3479aec7cf09f3f635