Add KM_MIN_MAC_LENGTH tag to HMAC and AES-GCM keys.

This makes Android Keystore add the KM_MIN_MAC_LENGTH tag to generated
and imported HMAC and AES-GCM keys. This tag specifies the minimum
length of the MAC/authentication tag authorized to be used for the
key.

For HMAC keys the minimum MAC length is set to the length of the
digest associated with the key (HMAC keys are authorized for exactly
one digest). For AES keys the minimum authetication tag length is set
to 96 bit. This is the minimum supported by Android Keystore's AES-GCM
implementation.

Bug: 22337277
Change-Id: Ic6e47cf084734d1592788dc58088889f7fff74eb