Renderscript: rename .rs extension to .rscript

Reserve .rs extension for Rust.

Bug: 137365032
Test: make checkbuild
Test: cd frameworks/compile/slang/tests
    ./slang_tests.py
Test: atest CtsRenderscriptTestCases
Test: CtsRsCppTestCases

Change-Id: Ic49fc7f5265d49cd4f8efd6a2b6a60b77bbd40dc
Merged-In: Ic49fc7f5265d49cd4f8efd6a2b6a60b77bbd40dc