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: I6e454f3c28c3014d0ac11a1cc3dd054e5ad2e98c
Merged-In: I6e454f3c28c3014d0ac11a1cc3dd054e5ad2e98c
(cherry picked from commit 81abf78dcfa140ea767f8c0b68072649d3c65bb9)
(cherry picked from commit 0b7e33a329a4919416824f8aded244d1ca628a05)