Use repo dependencies for sample builds

Using repository dependencies rather than file dependencies
for sample builds.

Specifically, the support library is pulled in directly now, so libs/
folders are no longer required.

Also removed superfluous defaultConfig{} and used
mavenCentral() for adding the Maven Central repository.

Change-Id: I4ce74031bdb171587086c17456d6cfc1ac635269

19 files changed