Add prebuilts/build-tools.

Bug: 27944293

default.xml

@@ -466,6 +466,7 @@
   <project path="pdk" name="platform/pdk" groups="pdk" />
   <project path="platform_testing" name="platform/platform_testing" />
   <project path="prebuilts/android-emulator" name="platform/prebuilts/android-emulator" groups="pdk-fs" clone-depth="1" />
+  <project path="prebuilts/build-tools" name="platform/prebuilts/build-tools" groups="pdk" clone-depth="1" />
   <project path="prebuilts/clang/darwin-x86/host/3.6" name="platform/prebuilts/clang/darwin-x86/host/3.6" groups="pdk,darwin" clone-depth="1" />
   <project path="prebuilts/clang/linux-x86/host/3.6" name="platform/prebuilts/clang/linux-x86/host/3.6" groups="pdk,linux" clone-depth="1" />
   <project path="prebuilts/clang/host/darwin-x86" name="platform/prebuilts/clang/host/darwin-x86" groups="pdk,darwin" clone-depth="1" />