bump minSdkVersion to 26 (Android 8.0)

[mahibi]

Percentage usage of android 7.0 for NC android files userbase is 0,45
For android 7.1 it's lower.

We have the rule to ditch android version lower than 0,5%.

ToDo:

• remove checks for android versions older than api level 26
• bump minSdkVersion to 26
• update documentation at https://docs.nextcloud.com/server/latest/admin_manual/installation/system_requirements.html#mobile-apps

[mahibi]

same for android talk: nextcloud/talk-android#4369