[video_player_avplay] Resolve drm manager proxy doesn't support multiple instances issue.

[xiaowei-guan]

https://github.com/flutter-tizen/plugins/blob/master/packages/video_player_avplay/tizen/src/drm_manager_proxy.cc#L9-L14

FuncDMGRSetData DMGRSetData = nullptr;
FuncDMGRGetData DMGRGetData = nullptr;
FuncDMGRSetDRMLocalMode DMGRSetDRMLocalMode = nullptr;
FuncDMGRCreateDRMSession DMGRCreateDRMSession = nullptr;
FuncDMGRSecurityInitCompleteCB DMGRSecurityInitCompleteCB = nullptr;
FuncDMGRReleaseDRMSession DMGRReleaseDRMSession = nullptr;

• If we create multiple Drm Manager, the new function pointer will overwrite the old ones, so we can extract a new class for drm manager proxy.
• Do not need dclose drm manager handler right away, which can cause coredump issue.

[xiaowei-guan]

Maybe we can change the drm manager proxy to a singleton.

[zg2nets]

Good job