GpgKeyLocator: Return a signing key for a user instead of the master key

GpgKeyLocator: Return a signing key for a user instead of the master key

Currently when a GPG key is looked up using a user identity the first
key from the keyring that has this user identity is returned.
The code was changed to instead return the first signing [S] key in this
keyring and only return the master key if no such signing key was found.
If the master key also does not have the signing flag set null is
returned instead.

Bug: 552288
Change-Id: I194862991d13c2c7ff34a60a54a227167f88f53b
Signed-off-by: Roan Hofland <roan.hofland@hotmail.com>