Some contributors who are building Fennec for the first time might already have copies of the Android sdk (and ndk) installed. We should provide some way of letting bootstrap know about these, in order to avoid unnecessary duplicate copies (which in turn saves disk space, and minimises annoyance). Maybe a prompt at the start of bootstrap asking for existing sdk and ndk locations? Ideally we'd support downloading any needed missing sdk package into an existing sdk installation. The ndk would have to be tested to ensure that it's a compatible version.
Or a simpler solution: ask whether or not the sdk/ndk should be installed during bootstrap (and warn the user that they'll need to install the appropriate sdk packages themselves, i.e. build tools X / sdk Y / support library repo / etc). This would still permit using bootstrap to install relevant system dependencies, without tying that to a potentially unneeded sdk download.
Even simpler: we should provide a step by step guide listing all the required dependencies, as an alternative to |mach bootstrap|, and a clear warning at the start of bootstrap explaining what bootstrap is going to do / how to avoid the mercurial/sdk installs. (Apparently mach bootstrap sets up mercurial too, but there are plenty of git users around too.)
Also: Often developers have set ANDROID_HOME and we could just use that if it is available.