Allow activated dots to keep their activated size

If mKeepDotActivated is true, dots will keep their activated size
after activation, and then reset their size when the pattern resets.

If mFadePattern is true (i.e. whenever keyguard challenge appears), the
dot stays activated until the next dot is reached.

This is a requested feature for wear devices.

Bug: 302189959
Test: set mKeepDotActivated to true and manually checked dot size
Change-Id: I1d51eb5aaa34466321c04ebb0aa543f8cdf501e0