vp9-svc: Set initial values for ext_buffer/flag indices.

Set initial values for these parameters in the vp9_init_layer_context().

This also fixes an issue in the svc-bypass mode when frame flags are
passed via the vpx_codec_encode().

Change-Id: I0968f04672f8d3d2fe2cea6b8a23f79f80d7a8b1