GCM: set default tag size to 12 bytes

According to RFC 5084, the default value of the GCM tag should be 12
octets (bytes). Change the default tag length from 0 to 12 to honor
this.

Bug: 22855843
Change-Id: I1ed16df24d0cfa9fff2593a3402c97faf913e05e
1 file changed