Unbreak 64bit builds

[NiLuJe]

The MultiDexApplication stuff changed in ABI 21+, which breaks if we use minSdk 21...

But, apparently, we can build with an NDKABI > minSdk, so, do that ;p.

(c.f., koreader/koreader#9632 (comment))

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This change is 

[pazos]

The unfortunate chain of events is:

1. Added support for 64 bit builds on this repo a couple of years ago
2. Enabled D8 to save a few KBs a few months later
3. D8 requires a "desugaring" library.
4. The desugaring library requires multidex support.

Here we are. Maybe for 200kb doesn't worth the hussle and we can get rid of multidex + desugaring?

In any case, LGTM

[NiLuJe]

2. Enabled D8 to save a few KBs a few months later

3. D8 requires a "desugaring" library.

4. The desugaring library requires multidex support.

Here we are. Maybe for 200kb doesn't worth the hussle and we can get rid of multidex + desugaring?

I have no clue what any of these things are ;o).

[pazos]

I have no clue what any of these things are ;o).

Me too.

But I know what they do: magic to strip a few kb out of the apk.

After a quick skim of various opensource apps I think we're fine with this PR alone

could you remove instead of comment?

[NiLuJe]

Reviewed 2 of 2 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @NiLuJe)