buildserver: standardize SDK and NDK install locations

/opt/android-sdk was chosen for a number of reasons:

• /opt is standardized for packages like the Android SDK, which has its own directory layout.
• android-sdk is used rather than the upstream directory name from the ZIP (e.g. android-sdk-linux) so that the path is the same on all platforms.
• On platforms without official /opt support (macOS, Windows, FreeBSD, etc.), it does not conflict with any existing system directory.

This should help with reproducibility since some tools like Python still include the build paths in the binaries. !919 (comment 578180986)

The default ANDROID_SDK_ROOT base dir of /opt/android-sdk is hard-coded in buildserver/Vagrantfile. The $ANDROID_HOME/ndk subdir is where Android Studio will install the NDK into versioned subdirs. https://developer.android.com/studio/projects/configure-agp-ndk#agp_version_41

This will also mean that the ci-images-* will have the same SDK/NDK paths as the production buildserver.

@obfusk @linsui @jspricke