android build updates

  - update to NDK r20
  - remove a few -I/-L paths we don't need
  - tacking $ndk_api onto -target has Clang set __ANDROID_API__;
    skcms doesn't really care but it's good hygiene
  - harmonize a few minor difference between 32/64-bit builds

Change-Id: I13f1580f16f28948c30e82d07ca97dd65d5f264f
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/225494
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
4 files changed