Use AndroidKeyStore to encrypt tokens.

[LossyDragon]

• Add: Crypto class that implements the AndroidKeyStore with an encrypt and decrypt functions.
• Add: CryptoTest class as an instrumentation test, making sure the implementation works.
• Add: Two new preferences to contain the encrypted tokens
• Change: Convert the old preferences to the new encrypted ones. Then clears them out.
• Change: 'remember me' to 'remember session'.
• Remove: password preference as we shouldn't need it (and it was always blank with my testing with 2FA and non-2FA).

Note: Having the alias in the repo shouldnt be an issue after researching. It only acts as a memory pointer for AKS. The keystore is all handled by the operating system.
Here is a good consice explanation for this: https://stackoverflow.com/a/49431434/13225929

Hopefully the build passes, as we have an instrumated test class.

[oxters168]

Great work!