Android is deprecating ndk.dir in favor of specifying exact NDK
version in Gradle configuration. Ensure that we can support multiple
NDKs, and link them into the location the Android Gradle Plugin expects.
androidenv did not previously write license files, which caused certain
gradle-based Android tools to fail. Restructure androidenv's list of
Android packages into a single repo.json file to prevent duplication
and enable us to extract the EULA texts, which we then hash with
builtins.hashString to produce the license files that Android gradle
tools look for.
Remove includeDocs and lldbVersions, as these have been removed
from the Android package repositories.
Improve documentation and examples.
We just need jdk for this script that helps set up toolchains. The
script might need to know about the target platform, but the interpreter
that runs it (or part of it) doesn't.
Move to a known version of the emulator. Each OS has a distinct
version... pick the one on Linux. A better solution would be to let
the user of `emulateApp` overload what the emulator version shall be
(and maybe it is already possible and I do not see it) - without need
to reproduce large portions of `default.nix`.
Using the previous emulator showed the following warning:
```
Your emulator is out of date, please update by launching Android Studio:
```
I am not aware of any reasons for not wanting the latest emulator (as
I expect it shall be compatible usually with more system images - not
less), so bump its default version.
Emulator release notes:
https://developer.android.com/studio/releases/emulator
