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: I0458009b8bb64a012fece364835d5b78d52e15e9
Merged-In: I0458009b8bb64a012fece364835d5b78d52e15e9