"Rename" external/libchromeos into external/libbrillo in repo manifest

libbrillo is a duplicate of libchromeos for the sake of "renaming" the
repo (which can't be done directly). So we just add libbrillo and remove
libchromeos from the manifest.
BUG: 25345697

default.xml

@@ -162,10 +162,10 @@
   <project path="external/kernel-headers" name="platform/external/kernel-headers" groups="pdk-cw-fs,pdk-fs" />
   <project path="external/ksoap2" name="platform/external/ksoap2" groups="pdk-cw-fs,pdk-fs" />
   <project path="external/libavc" name="platform/external/libavc" groups="pdk" />
+  <project path="external/libbrillo" name="platform/external/libbrillo" />
   <project path="external/libcap" name="platform/external/libcap" />
   <project path="external/libcap-ng" name="platform/external/libcap-ng" groups="pdk-cw-fs,pdk-fs" />
   <project path="external/libchrome" name="platform/external/libchrome" />
-  <project path="external/libchromeos" name="platform/external/libchromeos" />
   <project path="external/libcxx" name="platform/external/libcxx" groups="pdk" />
   <project path="external/libcxxabi" name="platform/external/libcxxabi" groups="pdk" />
   <project path="external/libdivsufsort" name="platform/external/libdivsufsort" groups="pdk" />