Fix Etheroll build, Move from NDK r17b to r17c

This is needed after fdroid/fdroidserver!637 was merged.
The error was:
```
CRITICAL: Android NDK version 'r17b' could not be found!
```
See:
https://f-droid.org/wiki/page/com.github.andremiras.etheroll/lastbuild_202117

Also bumps buildozer version so it doesn't fail silently, refs:
https://github.com/kivy/buildozer/issues/674

metadata/com.github.andremiras.etheroll.yml

@@ -19,7 +19,7 @@ Builds:
     sudo: apt-get install -y build-essential libffi-dev libltdl-dev python2.7 zlib1g-dev
     output: bin/etheroll-$$VERSION$$-release-unsigned.apk
     srclibs:
-      - buildozer@dfd2ceb
+      - buildozer@586152c
     prebuild:
       - sed -r "s:#?android.sdk_path =.*:android.sdk_path = $$SDK$$:" -i buildozer.spec
       - sed -r "s:#?android.ndk_path =.*:android.ndk_path = $$NDK$$:" -i buildozer.spec
@@ -27,7 +27,7 @@ Builds:
         -i buildozer.spec
       - pip install --user --upgrade $$buildozer$$ Cython==0.28.6
     build: PATH="$HOME/.local/bin:$PATH" buildozer android release
-    ndk: r17b
+    ndk: r17c
 
 MaintainerNotes: |-
     The build relies on buildozer master (post 0.39 release) which fixes issues