Mark keymint as unfrozen to manage keystore2's imports

We are now checking the imports of frozen versions of interfaces and
need mark keystore2 as `frozen: false` so the aidl_interfaces that
import it will import the latest unfrozen version.

Test: hal_implementation_test
Bug: 257338648
Change-Id: Ibcb151abd2fc13e3f7dfbcf515d0f62839d1caf9