Renderscript: change extension .rs 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: Ibfccb4f52b74f9ac95c373f48d775254ba77288d
Merged-In: Ibfccb4f52b74f9ac95c373f48d775254ba77288d
(cherry picked from commit 480864f1)
(cherry picked from commit f9b4f4b3)