ANDROID: arm64 defconfig / build config for cuttlefish

Add an arm64 defconfig with the Android and cuttlefish feature sets
merged in. This has been boot tested only on the QEMU virt model for
AArch64.

$ qemu-system-aarch64 -M virt -cpu cortex-a57 \
    -kernel arch/arm64/boot/Image.gz -serial stdio

Bug: 118442619
Change-Id: I99f3b78af85de8d051226f202351bd852a032248
Signed-off-by: Alistair Strachan <astrachan@google.com>