Enable multidex on Andoid versions prior to 21

[mueller-ma]

Fixes #617

Signed-off-by: mueller-ma [email protected]

Waiting for #611

[codecov-io]

Codecov Report

Merging #619 into master will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #619      +/-   ##
==========================================
- Coverage    31.8%   31.74%   -0.07%     
==========================================
  Files          64       64              
  Lines        4855     4855              
  Branches      655      655              
==========================================
- Hits         1544     1541       -3     
- Misses       3093     3095       +2     
- Partials      218      219       +1

Impacted Files	Coverage Δ
.../org/openhab/habdroid/ui/OpenHABWidgetAdapter.java	41.28% <0%> (-0.37%)	⬇️
...openhab/habdroid/ui/OpenHABWidgetListFragment.java	51.02% <0%> (-0.35%)	⬇️

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