KeyStore + target API 25

[ncioj10]

• update gradle wrapper and target API 25
• add KeyStoreProvider
• make use of hardware backed encryption through KeyStore

What did I do:
I created a KeyStoreProvider to encrypt non-password AES Keys before saving them.
For password Keys I create a random key which I save encrypted. After decrypting it again via the KeyStore I add it to the password on every initialization. This way non-password and password Keys can be protected with hardware backed security.

[scottyab]

@ncioj10 this looks really interesting. I'm struggling for time to review this PR. Please don't take the delay as a dis, i'm just busy with paid work and family.

[scottyab]

@ncioj10 This looks good, I'm in the process of splitting the SecurePreferences from the huge class it's ended up as into smaller/more flexible pieces. Once that's done i'll look to incorporate your KeyStore based changes. However likely it'll be in separate module to a allow it to be optional for other developers.

Don't worry about fixing any merge conflicts yet, as i say I'm in process of re-vamping the core code.

[ncioj10]

Sorry for the late reply, I was a bit overwhelmed by work. I think it also might be necessary to check for enivornment at some points in my code, so when you're finished with the refactoring give me a heads up and I'll fit my code accordingly.