buildserver: install default SDK packages using fdroid/sdkmanager

fdroid/sdkmanager provides a root of trust to verify all the packages it downloads, so it fully replaces what makebuildserver was doing.

This also upgrades to the latest version of Android SDK "tools". We have been frozen on tools_r25.2.5-linux.zip for the longest time because the buildserver provisioning required android update sdk to work. This replaces that with fdroid/sdkmanager, so the buildserver is free to have any version of "tools" installed, since none of the buildserver's setup processes need anything from "tools" any more.

Also:

• Stop installing android;m2repository by default. It is a huge package, it is rarely used, its not supported by fdroid/sdkmanager yet, and it is a pain to manage the install. If this breaks any app builds, the package can be installed as part of the build metadata.
• Stop installing ancient platforms: 2.1 (7), 2.2 (8), 2.3 (9) fdroid/sdkmanager has bugs with some of these, and some of these have been taken down. https://dl.google.com/android/repository/android-2.3_r01-linux.zip gives a 404.

@jspricke @licaon-kter @linsui