arm64-v8a image support

[dschmitt007]

Google has system-images;android-24;default;arm64-v8a image that would be nice to utilize for testing 64bit arm native code in a CI situation.

I took a shot at it, but can't expose the emulator to other machines.

Things to note:

the android wrapper is deprecated (sdkmanager and it's new package name scheme can replace it's use in Dockerfile, and avdmanager can replace it's use in entrypoint.sh) The android tool doesn't like android-24,sys-img-arm64-v8a-android-24 (maybe new issue)

emulator64-arm moved (it's in $ANDROID_HOME/emulator now). There is a binary "emulator" that will start the image, but it hooks up the crash monitor and starts a local adb and I haven't found the magic to expose it to other machines (running Docker for Mac, current arm32 images work fine) or start it bare (the old emulator wrapper script seems to be a binary now that does some stuff I haven't dug into.)