Add a tool for merging annotation.zip files

We now produce annotation.zip files individually for each module, but
for the public SDK it's easier to distribute a single zip file with
merged contents. Add a tool for merging these (simple) files.

Bug: 187397779
Test: unittest in cl
Test: diff tests in follow-on CL merging all module annotations.zip
Change-Id: Ic5bb94425dccafab43340805f73fafacab53cb28