rust: Refactor stripped output file path

Rust installed files reside in "$MODULE_OUT/stripped/" when they are
stripped, otherwise they reside in "$MODULE_OUT". However, other parts
of Soong assume that installed files are always in $MODULE_OUT
(cc_modules place *unstripped* files in $MODULE_OUT/unstripped).

This notably causes problems when adding Rust modules as test data in
AndroidMkDataPaths. When Rust modules are parsed by AndroidMkDataPaths,
if they are stripped then they incorrectly get installed as test data
with the path:
<install_root>/<relative_install_path>/stripped/file.

This CL refactors how we handle Rust stripped output such that the
installed file always resides in $MODULE_OUT.

Bug: 171710847
Test: Installed files now always reside in $MODULE_OUT
Change-Id: I53a6ff57a0a5a55cd95ea78ae592ce22abfa20c9