Generate merged txt file module with the checked in api files

This change introduces generating the genrule modules that generate the
merged txt files from the checked in api signature files of the
bootclasspath java_sdk_library modules.

Test: m nothing && inspect the ninja graph
Bug: 434908055
Flag: EXEMPT build refactor
Change-Id: I34199a02339307fa4e46668787bfbcebe6eea641