Sequences starting with intra-only frames previously resulted in undefined
behaviour with CONFIG_AMVR == 1, as seq_force_integer_mv was only read for
keyframes.

This patch makes changes as follows:

- The syntax element force_screen_content_tools has been added to the
  SequenceHeader struct, and is read and written correspondingly

- seq_force_integer_mv has been renamed to force_integer_mv and moved to the
  SequenceHeader struct, and is read and written correspondingly (provided that
  force_screen_content_tools != 0)

- The conditional reading/writing of allow_screen_content_tools now happens for
  every frame after reading/writing error_resilient_mode (CONFIG_OBU == 1) or
  the sequence header (CONFIG_OBU == 0)

- The conditional reading/writing of cur_frame_force_integer_mv now happens for
  every frame after reading/writing allow_screen_content_tools

BUG=aomedia:1048

Change-Id: I689476fc2fa781dc8ec6fc8da91926cc8cfd3dc2