RRO: Added partition policies for overlays

<overlayable> tags can now have policy elements that indicate which
partition the overlay apk must reside on in order to be allowed to
overlay a resource. This change only adds parsing of <policy> and
encoding of policy in the proto ResourceTable. A later change will add
the encoding of policy and overlayable in the binary APK.

<overlayable>
  <policy type="system|vendor|product|product_services|public" >
    <item type="string" name="oof" />
  </policy>
</overlayable>

Bug: 110869880
Test: make aapt2_tests
Change-Id: I8d4ed7b0e01f981149c6e3190af1681073b79b03