gomobile: make providing Android SDK optional
gomobile: make providing Android SDK optional

Currently there are no `aarch64-darwin` builds of Android SDK available.
For this reason attempts to build `gomobile` on that platform fail with:
```
No Android SDK tarballs are available for system architecture: aarch64-darwin
```

Signed-off-by: Jakub Sokołowski <1d3764b91b902f6b45836e2498da81fe35caf6d6@status.im>

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs