android: Lower minSdkVersion to 26, aka Android 8 Oreo

[gnprice]

Fixes #1898.

[gnprice]

@sphh Here's a zipfile of an APK built from this PR branch:
zulip-android8.zip

It should be just like the latest release (v30.0.265), except with the metadata saying it supports Android 8. As discussed in #1898, please try that out, and let us know if it works!

(You'll need to download the file first on your computer and unzip it; GitHub has a 25 MB limit on uploaded files, and wrapping it in zip is the way I got this file under that limit.)

[gnprice]

If your phone has an unusual CPU, that file might be unable to be installed in the first place. (That's the APK only for 64-bit ARM devices, which is almost all mobile devices these days.) If so I can post APKs for the other two possible CPU architectures and we can see which one works.

(Again that's in order to fit GitHub's upload-size limit. The APKs we post as releases — where I guess the same limits don't apply — include support for all three architectures.)