Use vp8_zero() or specify every element.

Cleans warning in Android build:
missing field 'deblocking_level' initializer
vp8_ppflags_t flags = {0};
                        ^
missing field 'sz' initializer
{0},                /* rc_twopass_stats_in */
  ^
missing field 'sz' initializer
{0},                /* rc_firstpass_mb_stats_in */
  ^
missing field 'layer_target_bitrate' initializer
}},
^
missing field 'deblocking_level' initializer
vp8_ppflags_t flags = {0};
                        ^
missing field 'mr_get_mem_loc' initializer
}
^

Change-Id: Iaedde9a77faac7a40316aee67d60d02ba7313500