locksettings: reduce visibility where unnecessarily broad

Reduce the visibility of methods and classes when possible, or add
@VisibleForTesting.

This is not comprehensive; it's just some cases I found in a quick scan
through the code.  I also didn't bother with some cases, like changing
public to package private in classes that are package private anyway.

Test: atest com.android.server.locksettings
Change-Id: I2fdef595d68316b4832948bbf85b70411e684703