Android: Add search feature

[kriegerse]

Hi,

I want to request a feature, which will enhance the usability for large collection of notes.

Additionally to #29 please make the notes fully searchable by headline and content (via index?) within the ownNote App.

I think this would be a great feature for many users.

Thank you!
Sebastian

[jan-gerard]

This is definitely a very important feature. I would like to be able to search within individual (big) notes, and in the whole list of notes or in a folder.