Improve decrypt MIME message process about flowcrypt-android HOT 2 CLOSED

The current logic:

1. We load MIME message to store locally
2. We encrypt the given input stream using KeyStoreCryptoManager(it uses Android Keystore). The last one uses AES/CBC/PKCS7Padding to encrypt the given input stream via CipherOutputStream. I didn't investigate performance at this point yet.
3. Later we open a locally saved MIME message(that was encrypted). For that, we use KeyStoreCryptoManager and CipherInputStream. And at this stage we have a performance issue. For example, on my device(not cheap) decryption of 19Mb(a large message with attachments. For example sent by FES) takes around 40 seconds. Slower devices have a worse situation. CipherInputStream is a reason. It has a very bad performance, ref 2.

So after some research, I decided to use PGPailnless in combination with KeyStoreCryptoManager

A new logic will look:

1. When a user adds a new account we create a random passphrase(PGP_PASS) via UUID.randomUUID()(it uses SecureRandom) and EC(curve25519) PGP private key(PGP_KEY) ecnrypted by this passphrase.
2. We encrypt the passphrase and private key by KeyStoreCryptoManager(Android Keystore) and save them in the local database in the account settings.
3. After that we use PGP_PASS and PGP_KEY to store a MIME message locally and read it.
4. These changes does great performance improvements. On my device decryption of 19Mb(a large message with attachments. For example sent by FES) takes around 3-4 seconds.

in short: instead of using Android Keystore to encrypt/decrypt a MIME message to work with them locally we will use Android Keystore to decrypt/encrypt PGP_PASS and PGP_KEY, and use them to encrypt/decrypt a MIME message to work with them locally.

@tomholub What do you think?

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• test migration from 1.4.8 to 1.4.9

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