fix: warnings in the Android NDK build

[supervacuus]

fixes #1085

It also includes the disabled warnings in the libunwindstack submodule (https://github.com/getsentry/libunwindstack-ndk/pull/9) and minor markdown fixes in the ndk readme.

skip-changelog

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 82.42%. Comparing base (f1699e1) to head (aaf666f). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1087 +/- ## ========================================== + Coverage 82.40% 82.42% +0.02% ========================================== Files 53 53 Lines 7751 7751 Branches 1216 1216 ========================================== + Hits 6387 6389 +2 + Misses 1253 1251 -2 Partials 111 111 ```

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