Specify default libvcx feature flags

[Patrik-Stas]

• fix build for libvcx ios/java artifacts (no features flags are selected for ios/java builds, will fail in runtime)
• fail on compile if no implementation is selected via feature flags

[codecov-commenter]

Codecov Report

Merging #908 (3b55aa1) into testing/remove-mixed-breed (f38fae4) will increase coverage by 0.00%.
The diff coverage is n/a.

@@                     Coverage Diff                     @@
##           testing/remove-mixed-breed     #908   +/-   ##
===========================================================
  Coverage                       43.37%   43.37%           
===========================================================
  Files                             438      438           
  Lines                           34777    34777           
  Branches                         7580     7580           
===========================================================
+ Hits                            15083    15086    +3     
+ Misses                          15231    15230    -1     
+ Partials                         4463     4461    -2     

Flag	Coverage Δ
unittests-aries-vcx	43.37% <ø> (+<0.01%)	⬆️

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see 3 files with indirect coverage changes