Support arm64-v8a

[tkada]

• Change Application.mk
  • Add arm64-v8a to APP_ABI
  • Change APP_STL from stlport_static to c++_static
• Add arm64-v8a and x86 plugin to Unity Project

[google-cla]

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[tkada]

@googlebot I signed it!

[belm0]

Unfortunately I no longer have admin access to this repo. You'd need to find someone in https://github.com/orgs/google/people to merge it.