feat: NDK

[bruno-garcia]

We need to solve a few things before merging this:

1. Builds go from 2-5 minutes to 10-20.
   Should we postpone merging while the main SDK is heavy under development and just bring master to this branch until then?
2. CI builds from API > 14.
   Introduce a build with/without NDK to run the lower API levels?

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@0e1fe4d). Click here to learn what that means.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##             master    #52   +/-   ##
=======================================
  Coverage          ?    38%           
  Complexity        ?    186           
=======================================
  Files             ?     51           
  Lines             ?   1413           
  Branches          ?     69           
=======================================
  Hits              ?    537           
  Misses            ?    850           
  Partials          ?     26

Impacted Files	Coverage Δ	Complexity Δ
...re/src/main/java/io/sentry/core/SentryOptions.java	72.6% <66.66%> (ø)	29 <1> (?)

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[tonyo]

@bruno-garcia Added a check, now we should only building NDK for a specific platform set via Travis environment variable ABI (looks like it's currently not used for anything else).
But just to keep in mind, when performing a release build, we should build all of them.