Make syspropRustGenRule work with absolute paths

This rule should be able to work with OUT_DIR set as absolute path,
which is used in "development/tools/ndk/update_ndk_abi.sh".

The problem appears if sysprop file itself is generated and
"syspropFile.String()" returns an absolute path, which is not accepted
as path component for "PathForModuleGen".

The fix is to use "android.GenPathWithExt" like in "syspropJavaGenRule".

Fixes: 328506541
Test: TreeHugger
Test: development/tools/ndk/update_ndk_abi.sh
Change-Id: Ia1e9a0ad51ebd957be97c48f5e1cf0e06a0da0ee