Handle the case when non-optional props have the same value

foo=true
foo=true
foo?=false

Consider the above case: Then the duplication of foo is allowed because
they have the same value (true). However, there was a bug that the
optional assirgnment foo?=false is left unmodified.

This fixes the bug by commenting such optional assignments.

Exempt-From-Owner-Approval: fixes a broken build

Bug: 117892318
Bug: 158735147
Test: atest test_post_process_props
Test: m out/target/product/vsoc_x86/vendor/build.prop for  cf_x86_auto

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

Merged-In: Iba9b61d9779d93e86d9bead2286f945f8d51ab1d
(cherry picked from commit 9a326367)
Change-Id: Iba9b61d9779d93e86d9bead2286f945f8d51ab1d