Rename core stubs to .txt stubs if applicable

In scope:
- core.current.stubs
- system modules generated for core's public stubs
- system modules generated for core's module_lib stubs

The system modules generated for core_platform api surface will be
handled in aosp/2514755

Test: go build ./java
Change-Id: I34134f79c8ae6e5b218d0b85553de5a748a8cc3f