Keystore 2.0: KeyProperties SignaturePadding is now public but hidden

In anticipation of the new Keystore 2.0 SPI we made this nested class
public (like its siblings) so that the new SPI which resides in a
different package may access it. It is hidden though because it does not
constitute public API surface.

Test: None
Bug: 171305684
Change-Id: I1dbe3d02c03f97f843813c26c16aaef7152ca478