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

Exempt-From-Owner-Approval: Clean CP
Change-Id: I33010b5604d8140f3a50845c16daff743504c8d0
Merged-In: I33010b5604d8140f3a50845c16daff743504c8d0
(cherry picked from commit fafc72bd)