Update Android toolchains and streamline process.

  - This updates to r10e / API v21 everywhere.
  - This has each host machine fetch the NDK,
    run make-standalone-toolchain.sh, and copy gdbserver itself.

(It will do all this once per $ARCH, which is a little inefficient, but it
washes out in steady state.)

BUG=skia:

Review URL: https://codereview.chromium.org/1385463002
2 files changed