Initial support for building against AOSP tree

Note that this support is highly experimental. Currently, a few issues
remain to be solved:

* Prebuilt libraries are needed for zxing and asn1bean. This will be
  pushed as a separate repository that can be included in the prebuilts/
  directory.
* The TelephonyManagerHiddenApi class needs to be excluded in AOSP
  builds
* In order to support multiple AOSP versions, the OpenEuiccService class
  needs to be turned into a custom interface with a default impl, and
  then we need to introduce impls for the system EuiccService depending
  on which version of AOSP we are building on. This could be kind of
  problematic, and I'm not sure whether we should do this yet.