Android SDK basic sample project and symlinked SDK Mentat binaries

[grigoryk]

Changes:

• Adds a sample android app which uses mentat SDK and opens a mentat db on launch
• Changes jniLibs/**/libmentat_ffi.so files within the SDK to be symlinks instead of binaries

This makes SDK development flow a bit nicer and provides a simple example project we can iterate on to test and showcase SDK features. Something like this:

• Compile mentat_ffi for all targets via scripts/android_build.sh (symlinks now resolve)
• Open the Mentat android project
• Make changes, try the sample project, make sure stuff works and APIs are all right, etc
• Make more changes to mentat-proper. scripts/android_build.sh x86, and try them out in an emulator. Simply relaunching the sample project should pick up the new version of the symlinked binary.
• Eventually, compile the final binaries scripts/android_build.sh, bump ext.library.version in build.gradle and upload to bintray: gradle bintrayUpload

This largely fixes #721 ("release" binaries are symlinked, so non-ideal for local dev).

[ncalexan]

This makes SDK development flow a bit nicer and provides a simple example project we can iterate on to test and showcase SDK features. Something like this:

Compile mentat_ffi for all targets via scripts/android_build.sh (symlinks now resolve)
Open the Mentat android project
Make changes, try the sample project, make sure stuff works and APIs are all right, etc
Make more changes to mentat-proper. scripts/android_build.sh x86, and try them out in an emulator. Simply relaunching the sample project should pick up the new version of the symlinked binary.
Eventually, compile the final binaries scripts/android_build.sh, bump ext.library.version in build.gradle and upload to bintray: gradle bintrayUpload

This largely fixes #721 ("release" binaries are symlinked, so non-ideal for local dev).

Can we get this fleshed out a little bit into a README somewhere in the FFI directory?

[grigoryk]

I'll add something useful to README regarding the flow.

I'll try to make the debug and release split work.

I agree regarding symlinking... I think it's a reasonable approach, given our audience (ourselves). Adding a caveat to README might help a bit.

[grigoryk]

Added some basic docs.

I'm going to punt the debug/release split to https://github.com/mozilla/mentat/issues/733. It's all right for now.