Create core_platform system modules from .txt files

Create a parallel set of java_system_modules that build using stubs from
.txt files. decodeSdkDep will be updated to use these system modules
behind a flag.

Since stub generation from .txt is not available at ToT yet, these
modules have been marked "enabled: false"

Test: None (will test when the enabled flag is turned on)
Change-Id: Ie9d465f5542a5430f03ba2e0861325011ac8e8c5