simplify Android NDK build config

If you just use the right compiler driver wrapper, recent NDKs automate
lots of the stuff we used to have to do ourselves.

Simplifying further, bump baseline ndk_api from 16 (Jellybean) on 32-bit
machines and 21 (Lollipop) on 64-bit to 21 across the board.  This makes
using libc++ a lot easier, as it hooks into a bunch of APIs that were
added in 18 and 21.  There's probably some way to work around this, but
the easiest thing is to just roll up.

Tested building {x86,arm}x{32,64} from a Linux host,
   and running  {    arm}x{32,64}.

Kind of flailing around in later CLs trying to get linking not to hang
on Windows.  I figure it's got something to do with lld?

Cq-Include-Trybots: skia.primary:Build-Win-Clang-arm64-Release-Android
Change-Id: I340b06fb9d372281146679d932417aaba3196045
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/225506
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Derek Sollenberger <djsollen@google.com>
5 files changed