Clear default-browser state when it's uninstalled

When the default browser is uninstalled, make sure to clean up that
bit of state in the package manager.  Failing to do this was giving
the settings UI problems.

Also, don't bother twiddling intent-filter verification state around
default-browser change: that state is intentionally ignored for
browser apps anyway.

Bug 35193618
Test: manual

Change-Id: I0cedc4efdc5a7e2aa0d322e9976b68e000e6eca7