Add intra_only and reset_frame_context flags

Added two flags to the frame header:

intra_only:
Signals that the frame is encoded using only INTRA
coding modes.

reset_frame_context:
Indicates that the coding context specified
in the frame header should be reset to default values before the
frame is encoded/decoded.

Change-Id: I182d46f1f84fb67a13c46ad767f246a38d7861a2