Session handling fixes

Fixed session handling issues, stopped storing session tokens in
a singleton, they are now stored in the database.
Fixed "duplicate finish request" log warnings.
Changed SVMPAppRTCClient toasts so that new toasts overwrite old
ones instead of queueing up.

Change-Id: Id98e492d0f0e813b80cb7d30750cc32b235ec470
Remove old dead jni files

These are from the old video code before we started using WebRTC fully
and somehow never got deleted previously.
Clean up and simplify ant/gradle builds and README

Ant automatically uses protobuf jar and SVMPProtocol.java from the
svmp-protocol-def project that is assumed to be checked out in a
directory parallel to the client.

Gradle now uses svmp-protocol-def as as subproject and doesn't need
any file copying tricks or ant custom rules to generate code straight
from the .proto definition file.

Change-Id: If9a4558510a3c85300f6d020330dbf54669c08da