I was very annoyed to find after like a year of using Plex + Nvidia SHield TV that a lot of my higher quality movies were being transcoded on the fly. I found through debugging logs this is because the SHield is being matched against the generic Android.xml. I adapted it and using options from other places to add in direct play support for pretty much everything. let me know your thoughts. File goes into /var/lib/plexmediaserver/Library/Application Support/Plex Media Server on my Fedora 25 server. Name it Shield.xml. I'm not 100% certain all of these formats are supported but so far it works pretty well and I've noticed a HUGE difference in the quality of the picture. Especially with items like HEVC 10bit color.
<?xml version="1.0" encoding="utf-8"?>
<Client name="SHIELD Android TV">
<Identification>
<Header name="User-Agent" substring="SHIELD Android TV" />
</Identification>
<!-- Author: Plex Inc. -->
<TranscodeTargets>
<VideoProfile container="mp4" codec="h264" audioCodec="aac" context="static" />
<MusicProfile container="mp3" codec="mp3" id="StereoMusicTranscodeProfile" />
<PhotoProfile container="jpeg" />
</TranscodeTargets>
<DirectPlayProfiles>
<VideoProfile container="mp4" codec="h264,mpeg4" audioCodec="aac" />
<VideoProfile container="mp4" codec="h264,mpeg4" audioCodec="aac,ac3,eac3,mp3,pcm" subtitleCodec="srt,ssa,ass"/>
<VideoProfile container="asf" codec="wmv3,wmv3,vc1" audioCodec="wmav2,wmav2,wmapro,wmavoice,pcm" subtitleCodec="srt,ssa,ass"/>
<VideoProfile container="avi" codec="h264,msmpeg4v3,mpeg4,mjpeg" audioCodec="mp3,ac3,eac3,dca,pcm" subtitleCodec="srt,ssa,ass"/>
<VideoProfile container="mkv" codec="h264,mpeg4,mjpeg,mpeg2video,vc1,hevc" audioCodec="mp2,mp3,ac3,eac3,dca,aac,pcm,vorbis" subtitleCodec="srt,smi,ssa,ass,subrip"/>
<VideoProfile container="mpeg" codec="h264,mpeg1video,mpeg2video" audioCodec="mp2,mp3,ac3,eac3,aac,pcm" subtitleCodec="srt,ssa,ass"/>
<VideoProfile container="flv" codec="h264" audioCodec="aac,ac3,eac3,mp3,pcm" subtitleCodec="srt,ssa,ass"/>
<VideoProfile container="mpegts" codec="h264,mpeg2video,vc1" audioCodec="mp2,mp3,ac3,eac3,dca,pcm" subtitleCodec="srt,ssa,ass"/>
<VideoProfile container="wtv" codec="mpeg2video" audioCodec="ac3,eac3,aac,pcm"/>
<VideoProfile container="3gpp" codec="h264,mpeg4" audioCodec="aac,he-aac"/>
<MusicProfile container="mp4" codec="aac" />
<MusicProfile container="mp3" codec="mp3" />
<MusicProfile container="flac" codec="flac" />
<MusicProfile container="ogg" codec="vorbis" />
<PhotoProfile container="jpeg,gif,bmp,png" />
</DirectPlayProfiles>
<ContainerProfiles>
<VideoContainer name="mp4">
<Limitations>
<Match name="part.optimizedForStreaming" value="1" />
</Limitations>
</VideoContainer>
</ContainerProfiles>
<CodecProfiles>
<VideoCodec name="*">
<Limitations>
<UpperBound name="video.width" value="1920" isRequired="true" />
<UpperBound name="video.height" value="1080" isRequired="true" />
<NotMatch name="video.anamorphic" value="1" />
</Limitations>
</VideoCodec>
<VideoCodec name="h264">
<Limitations>
<Match name="video.profile" list="baseline|main|high" />
</Limitations>
</VideoCodec>
<VideoAudioCodec name="aac">
<Limitations>
<UpperBound name="audio.channels" value="2" />
</Limitations>
</VideoAudioCodec>
</CodecProfiles>
</Client>