add repohooks

By default, no hooks are enabled, so this shouldn't impact people.
Bug: 28295071
Test: `repo upload` in a repo didn't blow up
Change-Id: I46a8476f7eaef80e68d9e7b3cc46741267150b67

default.xml

@@ -536,9 +536,12 @@
   <project path="tools/idea" name="platform/tools/idea" groups="notdefault,tools" />
   <project path="tools/loganalysis" name="platform/tools/loganalysis" groups="notdefault,tradefed" />
   <project path="tools/motodev" name="platform/tools/motodev" groups="notdefault,motodev" />
+  <project path="tools/repohooks" name="platform/tools/repohooks" />
   <project path="tools/studio/cloud" name="platform/tools/studio/cloud" groups="notdefault,tools" />
   <project path="tools/swt" name="platform/tools/swt" groups="notdefault,tools" />
   <project path="tools/test/connectivity" name="platform/tools/test/connectivity" groups="pdk" />
   <project path="tools/tradefederation" name="platform/tools/tradefederation" groups="notdefault,tradefed" />
 
+  <repo-hooks in-project="platform/tools/repohooks" enabled-list="pre-upload" />
+
 </manifest>