codec2: enc stat: add mapper for picture_type_t

Bug: 231851542
Test: atest android.media.codec.cts.VideoEncodingStatisticsTest

Change-Id: I855606e2b98b238d571d9b47d36db3f04e92ce06
Signed-off-by: Taehwan Kim <t_h.kim@samsung.com>
(cherry picked from commit 7c1a386a)
Merged-In: I855606e2b98b238d571d9b47d36db3f04e92ce06