Hide the public constructors for singleton classes

WebStorage and GeolocationPermissions are not intended for direct use
by application code. Existing APKs using this will still work (as well
as they ever could have), but this change will cause a compile break if they
move to SDK >= 16, which should be fixed by using getInstance() instead.

Bug: 6238010
Change-Id: I75789cc260c8fe005c42942bc81483193cc54f17