AAPT2: Allow any value type for <item> without format attr

TO bring AAPT2 behavior in-line with AAPT, <item> has a default
format of "any", and only becomes restricted with an explicit format
attribute.

Bug: 62260121
Test: make aapt2_tests
Change-Id: Ife416f520e6c2710bb30e3ba3f2d4463794bfa06