(WIP) Change password when key is stripped #1692

Approach:
Find the first unstripped secret key and use it for passphrase verification
All unstripped keys will have their passphrase changed to new passphrase, if possible.

Current Progress:
Changing the passphrase of keys works fine.
Refactoring to combine "modifySecretKeyring" and newly added method, "modifyKeyRingPassword"
may be possible if given the go-ahead.

OpenKeychain/src/main/res/values/strings.xml

@@ -1063,6 +1063,7 @@
     <!-- modifySecretKeyRing -->
     <string name="msg_mr">"Modifying keyring %s"</string>
     <string name="msg_mf_divert">"Will use Security Token for crypto operations"</string>
+    <string name="msg_mf_error_all_keys_stripped">All keys are stripped!</string>
     <string name="msg_mf_error_divert_newsub">"Creation of new subkeys is not supported for primary keys on Security Tokens!"</string>
     <string name="msg_mf_error_divert_serial">"The serial number of a key on Security Tokens must be 16 bytes! This is a programming error, please file a bug report!"</string>
     <string name="msg_mf_error_encode">"Encoding exception!"</string>
@@ -1077,6 +1078,7 @@
     <string name="msg_mf_error_null_expiry">"Expiry time cannot be "same as before" on subkey creation. This is a programming error, please file a bug report!"</string>
     <string name="msg_mf_error_noop">"Nothing to do!"</string>
     <string name="msg_mf_error_passphrase_master">"Fatal error decrypting master key! This is likely a programming error, please file a bug report!"</string>
+    <string name="msg_mf_error_passphrases_unchanged">"Error changing all passphrases!"</string>
     <string name="msg_mf_error_pgp">"Internal OpenPGP error!"</string>
     <string name="msg_mf_error_sig">"Signature exception!"</string>
     <string name="msg_mf_error_sub_stripped">"Cannot modify stripped subkey %s!"</string>