Previously, we were mapping certain named platforms
(e.g. `android-stable`) to their corresponding version.
this had two problems:

1. The version could become out of date. For instance, we had
   mapped `android-stable` to version 24, but the stable version
   is now 27.
2. The list of possible named versions wasn't comprehensive.
   Some Android SDKs just list the platform as `stable`, or
   `experimental`, etc.

This change updates the platform version detection to use
the `build.prop` file that exists in the platform directory
(only for cases where the version number is not encoded into
the directory name).