AAPT2: Respect format attribute of <item> tag

An <item> is a general tag that can override certain behavior. For
instance, this is allowed:

    <item name="foo" type="integer" format="float">0.4</item>

Even though without the format attribute, this would be illegal.

Change-Id: I8133ce59e14719a70d7476a1464c3f564c435289