AAPT2: Revert warning when merging packages

A compiled ResourceTable may contain multiple packages.
When referencing a framework attribute, for instance,
the compile phase has no idea if that attribute is a USE
or a DECLARATION, like so:

  <declare-styleable name="Foo">
    <item name="android:id" />
  </declare-styleable>

If this file is compiled with the package 'android', then this
is a declaration. If it is compiled with the package 'com.foo',
this is a reference that must resolve at link time.

Since this is common, warning that the package 'android' is being
ignored during the merge phase is incorrect behaviour.

The warning was antroduced in ag/1363992

Change-Id: I09254d79ac379439face177eb724df882d1d0ce1