Add project platform/prebuilts/runtime to the manifest.

Project platform/prebuilts/runtime 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: 147283805

Change-Id: I2e7a47e6e62fa13c51c9e06810012990456ca54b

default.xml

@@ -713,6 +713,7 @@
   <project path="prebuilts/python/linux-x86/2.7.5" name="platform/prebuilts/python/linux-x86/2.7.5" groups="linux,pdk,pdk-cw-fs,pdk-fs" clone-depth="1" />
   <project path="prebuilts/qemu-kernel" name="platform/prebuilts/qemu-kernel" groups="pdk" clone-depth="1" />
   <project path="prebuilts/remoteexecution-client" name="platform/prebuilts/remoteexecution-client" groups="pdk" clone-depth="1" />
+  <project path="prebuilts/runtime" name="platform/prebuilts/runtime" groups="pdk" clone-depth="1" />
   <project path="prebuilts/rust" name="platform/prebuilts/rust" groups="pdk" clone-depth="1" />
   <project path="prebuilts/r8" name="platform/prebuilts/r8" groups="pdk" clone-depth="1" />
   <project path="prebuilts/sdk" name="platform/prebuilts/sdk" groups="pdk" clone-depth="1" />