Fix NDK library sysroot arrangement.

We've been fixing this up on the NDK build side for ages. The
platforms/ subdirectory is a legacy thing from very old NDKs. Fix the
install paths to match what the NDK actually uses. This both lets us
delete some code on the NDK side and also makes the platform's output
easier to consume directly without having to wait for an NDK release
(or even a canary).

Bug: None
Test: treehugger
Test: inspected out/soong/ndk/sysroot
Test: build/soong/scripts/build-ndk-prebuilts.sh, imported into NDK
Change-Id: I3c4e0ce6d7960ae0138d1cf803755e4f1e575631