bump NDK packages to r19-beta2

Originally this started as a bump to r18, the latest stable release.
That's why each packages goes up two versions, from r17 -> r18 ->
r19-beta2.

But our ASAN bots started failing with r18, and despite quite a bit of
investigating, I've been unable to find anything wrong with our code.

Luckily, r19 is coming soon, and it looks like its beta2 works fine.
I think we should just skip r18.

Original change's description:
> bump NDK packages to r18
>
> Newer Clang, GCC is gone.
>
> Clang now supports half-float math on ARMv8, which is pretty neat.
>
> They've dropped support for everything below NDK version 16,
> which happens to be what we target for 32-bit ARM.
>
> Change-Id: Idd1b1b557c5ecaabec4040026fd2ad5adfee5ee7
> Reviewed-on: https://skia-review.googlesource.com/157260
> Reviewed-by: Derek Sollenberger <djsollen@google.com>
> Commit-Queue: Mike Klein <mtklein@google.com>

Cq-Include-Trybots: skia.primary:Test-Android-Clang-Pixel-CPU-Snapdragon821-arm64-Release-All-Android_ASAN
Change-Id: I0736558fc9b732113854b1b295642e1708b04581
Reviewed-on: https://skia-review.googlesource.com/c/157761
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
10 files changed