Replace internal APIs in android.webkit.UserPackage.

UserPackage is only consumed by other WebView-related code (which does
not care about the UserInfo information), and by Settings (which can
query UserInfo itself).

To make it easier to move this to a mainline module, remove references
to the internal UserInfo from the API surface and instead just store a
UserHandle which is a public API.

Replace use of the internal getPackageInfoForUser with equivalent public
APIs.

Bug: 310653407
Test: atest WebViewUpdateManagerTest
Flag: NONE cannot flag changed types in method signatures
Change-Id: I119862b927c30b43e4309828f8fdf42a826a0cb5