Don't append build directory to the output path twice.

Soong already uses build directory as a part of the generated output
path. Besides, trying to append it second time breaks if the build
directory is an absolute path.

Test: frameworks/rs/build_rs.py  (uses absolute build directory path)
Test: walleye-userdebug boots

Bug: 152235239
Change-Id: Ie03c9e688013b9a3e6bc859c936d89c538ded76a