Allow code generation without android SDK/NDK

[sonicdebris]

We have the same issue mentioned in #107 (specifically: #107 (comment)).

In short, some of our developers working with the generator are not android developers, and would rather not have to set-up the android SDK/NDK to use djinni, considering those are only needed to build the android examples.

I know nothing about Bazel, but still I decided to try and find a solution to this. I started investigating a way to conditionally trigger the android repo rules for the examples only, when I found this discussion:
bazelbuild/bazel#14260

From there, I understand that the rule should only be applied in case one tries to build a target that requires the android SDK, but apparently that is not the case yet. In the same discussion, I found a suggestion for a workaround, which amounts to only defining the android_sdk_repository and android_sdk_repository rules if the environment variables for the paths to the SDK/NDK are set:
envoyproxy/envoy-mobile#2039

I implemented the same, with a trivial tweak, which is printing a warning in case the environment variables are not set, to counterbalance the fact that the build failure message for the android-app and android_sdk_repository is now less explicit.

It seems to work fine, hopefully it's not too hacky and can be useful upstream.

[li-feng-sc]

Thanks! This is something I have wanted for a long time. I'll need to do some tests with Snap's codebase and then get back to you.

[li-feng-sc]

@sheldonneuberger-sc we use a trimmed down version of the workspace file, so I believe this will not impact Snap code.

[li-feng-sc]

I tested this on my laptop and it works very well! Thank you @sonicdebris !

[sheldonneuberger-sc]

Nice, thanks for fixing that.

[nmtitov]

This is very good idea, I like it. I mostly do iOS development and having Android dependency installed is really annoying for me.