AAPT2: include package name in styleable children

When writing partial R files, also include the package of the styleable
child:
<declare-styleable name="ds1">
    <attr name="font"/>
    <attr name="android:font"/>
    <attr name="tools:font"/>
    <attr name="com.foo.bar:font"/>
</declare-styleable>
will produce:
default int styleable ds1_font
default int styleable ds1_android_font
default int styleable ds1_tools_font
default int styleable ds1_com_foo_bar_font

Bug: 73927419
Test: manual, see description
Change-Id: Ica493408c7c6d9e7fae2b693c96fbc20a723ce08