Remove the java.time.Instant dependancy

[martinbonnin]

The current code uses java.time.Instant which is only available on java 8 and android 26+.

[martinbonnin]

This is discussed already in #21.

I'm keeping this issue open as a pointer for newcomers like me.

[jawadst]

@martinbonnin I have a working version for Android here: https://github.com/didomi/Consent-String-SDK-Android
Waiting for it to be moved to the IAB repo

[martinbonnin]

@jawadst Thanks ! Ultimately, I ended up writing a shortened version of it in kotlin.

It only supports writing and is not very configurable but it works so far => https://gist.github.com/martinbonnin/70651cf84eb1512d15a87dd290e6afde