add os(Android) compiler conditions

[gr-markin]

This PR adds the compiler conditions required to build the library on Android. While PR#1708 also includes compiler conditions along with other code changes, this PR contains only the minimal changes needed for Android, which will hopefully simplify its acceptance.

• CONTRIBUTING: You have read https://github.com/groue/GRDB.swift/blob/master/CONTRIBUTING.md
• BRANCH: This pull request is submitted against the development branch.
• DOCUMENTATION: Inline documentation has been updated.
• DOCUMENTATION: README.md or another dedicated guide has been updated.
• TESTS: Changes are tested.
• TESTS: The make smokeTest terminal command runs without failure.

[groue]

Hello @gr-markin.

Some work on Linux was performed in #1825 by @thinkpractice. An excellent work actually because instead of "just make it compile", features that had been disabled by previous "just make it compile on Linux" contributions were restored. That level of care is well appreciated.

This pull request is obsolete. For further work on Android, please check other and recent pull requests and contact the involved contributors.

Thank you!