Android support

Hi! I see two possible uses of this library on Android:
- By some JVM apps, through their own rust library (e.g. game engines)
- By cross-platform term apps, through Termux (e.g. [jmacdonald/amp](https://github.com/jmacdonald/amp/issues/208))

The latter probably can be done by simply running process '[termux-clipboard-get](https://wiki.termux.com/wiki/Termux-clipboard-get)/[set](https://wiki.termux.com/wiki/Termux-clipboard-set)'. About the first case — there is [an easy way](https://developer.android.com/guide/topics/text/copy-paste) in SDK to use the clipboard, however, it needs to be used through JNI. [This crate](https://crates.io/crates/jni) is used by [Servo](https://github.com/servo/servo) among others, so I think it'll be fine to use.

Things to think about:
- [ ] Can the Termux case be covered by the JNI thing?
- [ ] How to handle different Android SDKs?
- [ ] How to test?