BUILD_BROKEN_DUP_SYSPROP as escape hatch for the new sysprop restriction

As the final step for the refactoring of sysprop configuration, this
change adds BUILD_BROKEN_DUP_SYSPROP which is the escape hatch for
the new restriction. When it is turned on, the new syntax `a ?= b`
collapses to the old syntax `a = b`, duplicated assignments are allowed,
and the dups are resolved following the legacy rule of preferring the
first.

This change also summarizes all the user-facing changes to the Change.md
file.

Lastly, post_process_prop.py is refactored to accept new argument
'--allow-dup' which when turned on allowes duplicated sysprops.

Bug: 117892318
Bug: 158735147
Test: atest --host post_process_prop_unittest

Exempt-From-Owner-Approval: cherry-pick from master

Merged-In: I7bdfffd47d50aad66a78e28a30c3dad7ebac080c
(cherry picked from commit b302cdf6a4)
Change-Id: I7bdfffd47d50aad66a78e28a30c3dad7ebac080c

tools/test_post_process_props.py

@@ -226,5 +226,24 @@ class PropListTestcase(unittest.TestCase):
         # since they have the same value
         self.assertTrue(override_optional_props(props))
 
+  def test_allowDuplicates(self):
+    content = """
+    # comment
+    foo=true
+    bar=false
+    qux?=1
+    foo=false
+    # another comment
+    foo?=false
+    """
+    with patch("post_process_props.open", mock_open(read_data=content)) as m:
+      stderr_redirect = io.StringIO()
+      with contextlib.redirect_stderr(stderr_redirect):
+        props = PropList("hello")
+
+        # we have duplicated foo=true and foo=false, but that's allowed
+        # because it's explicitly allowed
+        self.assertTrue(override_optional_props(props, allow_dup=True))
+
 if __name__ == '__main__':
     unittest.main(verbosity=2)