android-toolchain needs to sensibly reset SDK & NDK

[jonpryor]

Context: #1112 (comment)

The Android SDK & NDK install is "forward only" (see b28856f).

Steps to Reproduce

1. Create a new branch, and on that branch bump the Android NDK dependencies to something "newer".
2. Build the branch from (1), thus installing the "newer" NDK into $HOME/android-toolchain/ndk
3. Checkout a different branch -- one that uses an "older" NDK -- and rebuild.

Expected Behavior

The Android NDK should be re-extracted, so that $HOME/android-toolchain/ndk matches what the current branch requires.

Actual Behavior

The Android NDK is not touched at all.

[pjcollins]

I think this was resolved as part of #3051 and #3213