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.

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 dex2oat)
Test: env OUT_DIR=out-tools prebuilts/build-tools/build-prebuilts.sh
  on the aosp-build-tools branch
Change-Id: I38317f2d5e5cf6822f87daae25745767c8a3656c
diff --git a/java/app.go b/java/app.go
index c59047d..4f06087 100755
--- a/java/app.go
+++ b/java/app.go
@@ -27,6 +27,7 @@
 
 	"android/soong/android"
 	"android/soong/cc"
+	"android/soong/dexpreopt"
 	"android/soong/tradefed"
 )
 
@@ -850,6 +851,7 @@
 	android.ModuleBase
 	android.DefaultableModuleBase
 	prebuilt android.Prebuilt
+	dexpreopt.DexPreoptModule
 
 	properties   AndroidAppImportProperties
 	dpiVariants  interface{}