AAPT2: Add <overlayable> tag support

This doesn't actually do anything yet, but makes sure
it is not a syntax error and allows teams to start marking
their resources as overlayable.

The syntax form marking a set of resources as overlayable
looks like:

  <overlayable policy="system">
    <item type="string" name="foo" />
    <item type="style" name="bar" />
  </overlayable>

Currently, the only supported policy is "system", meaning only
the system will be able to overlay the resources. Leaving
out the policy attribute defaults to "system".

Bug: 64980941
Test: make aapt2_tests
Change-Id: Ied7a9ddae87a4a0af6a0f4d1c213bfce8a0ed612
3 files changed