Reland: Get the dex2oat host tool path from module dependency on the

binary module.

This uses the Once cache for GlobalSoongConfig to propagate the dex2oat
path from a module dependency to the singletons (both the one that
writes out dexpreopt_soong.config and the one that creates the
dexpreopted boot images). Unless dexpreopting is disabled altogether
through DisablePreopt in dexpreopt.config, that means:

- We must ensure at least one module registers a dex2oat tool
  dependency and resolves a GlobalSoongConfig using it, or else the
  singletons fail. That means we litter dex2oat dependencies in java
  modules even when they won't get dexpreopted and hence don't really
  need them.

- We still assume there's at least one java_library or android_app in
  the build.

This relands https://r.android.com/1205730 without changes - the
necessary fixes are in the child CLs.

Bug: 145934348
Test: m
  (check that out/soong/dexpreopt_soong.config points to dex2oatd64)
Test: env USE_DEX2OAT_DEBUG=false m
  (check that out/soong/dexpreopt_soong.config points to dex2oat64)
Test: env OUT_DIR=out-tools prebuilts/build-tools/build-prebuilts.sh
  on the aosp-build-tools branch
Change-Id: I66661711b317d1e4ec434861982919bdde19b575

dexpreopt/Android.bp

@@ -4,6 +4,7 @@ bootstrap_go_package {
     srcs: [
         "config.go",
         "dexpreopt.go",
+        "testing.go",
     ],
     testSrcs: [
         "dexpreopt_test.go",