Support zstd on Android

[mhsmith]

Feature or enhancement

As see on the buildbot, it's currently being omitted:

checking for libzstd >= 1.4.5... no

In the current build system, this will require the following:

• Add a libzstd release in https://github.com/beeware/cpython-android-source-deps.
• Update android.py to download it before running configure.

Related:

• https://github.com/beeware/cpython-apple-source-deps/issues/60