Depend on Keystore2 AIDL via default

This allows for easier bumping of the KeyMint/Keystore2 version level.

At the moment this change should have no effect: the same dependency
is used, just reached via a default rather than explicitly.

However, when the Keystore2 version increases in the near future, using
this default should mean that no change is needed here: the default
definition will change to -V2 and this will be referenced here.

Test: TreeHugger
Change-Id: I7cec2cd8fac137e5ac7e95db06e738e94a3cc567