android: improve vp8 video streaming

During a VP8 video call on the android client, the peer video freezes.
Optimizes android video buffers using an array instead of a vector. Lets
VP8 decide when to send a keyframe.

Change-Id: I19893c2ac080782daee72958fb6c174eea42037a
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>