examples/android/ReadMe: Instruct users to run "mvn install" ("mvn package" is no longer enough).
examples/android/ReadMe: Instruct users to run "mvn install" ("mvn package" is no longer enough).

"mvn package" used to be enough when we referenced the JAR file
directly, but now we use a Maven package reference.

dropbox/dropbox-sdk-java,dropbox/dropbox-sdk-java,dropbox/dropbox-sdk-java,dropbox/dropbox-sdk-java