auto-download missing NDKs if they're known and can be verified

This adds a new method to automatically download and install the NDK version as specified in the ndk: field in a build, if it is not already present. This is meant to run only in with fdroid build --onserver so in the buildserver VM, fdroid build CI job, and issuebot runs. This also removes all but 3 NDK versions from the buildserver VM (r10e special case, latest LTS version, and latest version). Right now, this just includes the checksums directly in fdroidserver/common.py to keep it as simple as possible. That means if a new NDK version is released and it is not yet included there, this will fail to install it. Ultimately, the checksums should come from a dedicated source #737 (closed), like android-sdk-transparency-log.

Once this is proven and handling NDK r10e is sorted out, then the NDK provisioning in the buildserver could be entirely removed. #902 (closed)

refs #517 closes #717 (closed) closes #369 (closed)