aapt2: add 'dump overlayable' command

Add a command to print a resource table's <overlayable> resources. Given
the following input

  <overlayable name="TestResources">
    <policy type="system">
      <item type="string" name="a" />
    </policy>
    <policy type="sytem|vendor">
      <item type="string" name="b" />
      <item type="string" name="c" />
    </policy>
  </overlayable>

aapt2 dump overlayable will produce

  name="TestResources" actor=""
    policies="system"
      string/a
    policies="system|vendor"
      string/b
      string/c

Bug: 120609160
Test: manual (aapt2 dump overlayable $(gettop)/frameworks/base/cmds/idmap2/tests/data/target/target.apk)
Change-Id: I21041e6169c62d01f1a469624911ce7cad3e18a8