Use separate directories for separate versions of the
SDK, NDK, and toolchain, and use symlinks to point to the current
versions. This is more robust for a few reasons:
 - Partially-completed upgrades to new versions won't touch
   existing versions on the disk
 - The symlinks aren't updated until the respective new version is
   completely installed, allowing for more transactional updates.
 - The symlinks also allow for constant paths in the bash_profile file
   and the buildbot config, which means less moving parts to break.
   In particular, the relevant buildbot config is on the master, and
   using symlinks makes it unnecessary to use Salt orchestration to
   gate changes to the master buildbot config on changes on the cross
   builders.

Ideally, these states would also use file.directory with clean: True
to clean out old versions of the SDK, NDK, and toolchain, but I wasn't
able to get this working properly yet (the just-downloaded files would
keep getting cleaned away).

This also eliminates our use of cmd.wait, which was recently put on the
deprecation path in Salt, and replaces it with cmd.run + creates: True.