Renderscript: rename .rs extension to .rscript

As far as I can tell, this is the only place where soong is used
to build renderscript.

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

Change-Id: I10a23ceee696df7fe363cc5d825e45100565cbdf