Steps to reproduce:
1. Open QA attached project
2. Switch to Android platform
3. Deploy the app to a device and it let run for a bit
4. After restarting the video 10~20 times the VideoPlayer.prepareCompleted misfires resulting in failure to launch the video again

Expected behaviour: VideoPlayer.prepareCompleted fired every time the video is ready
Actual behaviour: VideoPlayer.prepareCompleted sometimes doesn't fire

Reproduced on: 2017.2.2p1, 2017.3.1p4, 2018.1.0b11, 2018.2.0a4

DUT:
Reproduced:
0079 Samsung SM G930F, OS:7.0, CPU:arm64-v8a, GPU:Mali-T880,
0143 Samsung SM G950F, OS:7.0, CPU:arm64-v8a, GPU:Mali-G71,
0076 Xiaomi MI MAX, OS:6.0.1, CPU:arm64-v8a, GPU:Adreno (TM) 510,
5022 Xiaomi MI PAD, OS:4.4.4, CPU:armeabi-v7a, GPU:NVIDIA Tegra,
VLNQA00025 Xiaomi MI 5s, OS:6.0.1, CPU:arm64-v8a, GPU:Adreno (TM) 530,

Not reproduced:
0144 Samsung SM G950U, OS:7.0, CPU:arm64-v8a, GPU:Adreno (TM) 540,
Caused by https://issuetracker.unity3d.com/issues/android-videoplayer-dot-prepare-with-multiple-videos-freezes-the-application-ndkmediacodec-sf-error-code-38

Fixed in: 2017.4.5f1, 2018.1.3f1, 2018.2.0b3, 2018.3.0a1