minui: add ability to synchronize current key state

If a key is down prior to the time of initialization, we would not get the
down event for the key, and thus think that the key is not pressed.

Add an interface that allows one to provide a callback to execute
on all keys that are currently down.

Change-Id: I2a4096c0cb4c7c7a9a80d207835f168a0b418413
Signed-off-by: Dima Zavin <dima@android.com>