Add abi-compliance-checker and abi-dumper.

Test: repo sync -j64, I can see abi-compliance-checker and abi-dumper in
platform/external.
Change-Id: I76ce1b9bd8ef15aced31ac2cbf936f1dfde11ef5

default.xml

@@ -76,6 +76,8 @@
   <project path="device/sample" name="device/sample" groups="pdk" />
   <project path="docs/source.android.com" name="platform/docs/source.android.com" groups="pdk-cw-fs,pdk-fs" />
   <project path="external/aac" name="platform/external/aac" groups="pdk" />
+  <project path="external/abi-compliance-checker" name="platform/external/abi-compliance-checker" groups="pdk" />
+  <project path="external/abi-dumper" name="platform/external/abi-dumper" groups="pdk" />
   <project path="external/adt-infra" name="platform/external/adt-infra" groups="adt-infra,notdefault,pdk-fs" />
   <project path="external/android-clat" name="platform/external/android-clat" groups="pdk" />
   <project path="external/androidplot" name="platform/external/androidplot" groups="pdk" />