Add jni

[mahesh-hegde]

• Adds JNI to FFI plugin, along with android support

• There are several generated files which can be hard to review.

• To verify reproducibility

  • run tool/gen_aux_methods.dart in jni/ folder

  • run dart run ffigen --config ffigen.yaml to generate bindings to jni.h ; The header file is copied from Android NDK plus some annotations for param names in function pointer fields. The ffigen currently links to my fork of ffigen with a jni-specific patch, on which changes from upstream ffigen should be easy to verify. I think that's better than committing all vendored files in the same commit.

Marking it as draft to get the CI right, eventually.

[coveralls]

Pull Request Test Coverage Report for Build 2635450534

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 2623143830:	0.0%
Covered Lines:	1
Relevant Lines:	1

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💛 - Coveralls

[mahesh-hegde]

Closing this PR in favor of new branch.