Add project device/generic/art to the manifest.

Project device/generic/art is used in some ART builds. The main
motivation to add it to the main Android manifest is to enable
automerger paths on this project.

Test: Validated with validate_repo_branch.par
      (see go/mani-verify#using-validate-repo-branchpar):
        /google/data/ro/projects/android/android-build/validate_repo_branch.par \
          --target_repo=android --branch=master \
          --local_manifest .repo/manifests/default.xml
Bug: 147283354

Change-Id: Iee92f1a5db4647f6a6d4a21fdda1646a8d8ca8e0

default.xml

@@ -39,6 +39,7 @@
   <project path="device/common" name="device/common" groups="pdk-cw-fs,pdk" />
   <project path="device/generic/arm64" name="device/generic/arm64" groups="pdk" />
   <project path="device/generic/armv7-a-neon" name="device/generic/armv7-a-neon" groups="pdk" />
+  <project path="device/generic/art" name="device/generic/art" groups="pdk" />
   <project path="device/generic/car" name="device/generic/car" groups="pdk" />
   <project path="device/generic/common" name="device/generic/common" groups="pdk" />
   <project path="device/generic/goldfish" name="device/generic/goldfish" groups="pdk" />