Update port for Ubuntu Touch 20.04 on Xiaomi Mi 6

[kuailexs]

Recently,
I'm trying to update the mi6 port for Ubuntu Touch 20.04.

known issue:

1. rtc time

In other respects, it's the same as Verevka's port.
see : https://forums.ubports.com/topic/4671

merge requests :
https://gitlab.com/ubports/porting/community-ports/android9/xiaomi-mi-6/xiaomi-sagit/-/merge_requests/1
https://gitlab.com/ubports/infrastructure/devices.ubuntu-touch.io/-/merge_requests/588

For VoLTE on Mi 6 ：unavailable

proprietary blobs for Android 9
radio interface version is 1.1
ims version is 1.0

Use this patch to load the ims library.

ofono IpMultimediaSystem is not registered

phablet@ubuntu-phablet:~$ gdbus call --system --dest org.ofono --object-path /ril_0 --method org.ofono.IpMultimediaSystem.GetProperties

({'Registered': <false>, 'Registration': <'auto'>, 'VoiceCapable': <false>, 'SmsCapable': <false>},)

[Keneda]

@kuailexs
Please edit your image to hide personal infos like mac addresses, IMEI , serials and so on.