Support optional prop assignments

This CL adds a number of changes to make the assignment of system
properties to be less confusing.

1. Added `a ?= b` syntax, which is called optional prop assignments. The
prop `a` gets the value `b` only when there is no non-optional prop
assignment for `a` such as `a = c`. This is useful for props that
provide some reasonable default values as fallback.

2. With the introduction of the optional prop assignment syntax,
duplicated non-optional assignments is prohibited; e.g., the follwing
now triggers a build-time error:

a = b
a = c

, but the following doesn't:

a ?= b
a = c

Note that the textual order between the optional and non-optional
assignments doesn't matter. The non-optional assignment eclipses the
optional assignment even when the former appears 'before' the latter.

a = c
a ?= b

In the above, `a` gets the value `c`

When there are multiple optional assignments without a non-optional
assignments as shown below, the last one wins:

a ?= b
a ?= c

`a` becomes `c`. Specifically, the former assignment is commented out
and the latter is converted to a non-optional assignment.

3. post_process_props.py is modified so that when a prop assignment is
deleted, changed, or added, the changes are recorded as comments. This
is to aid debugging. Previously, it was often difficult to find out why
a certain sysprop assignment is missing or is added.

4. post_process_prop.py now has a unittest

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

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

Merged-In: I9c073a21c8257987cf2378012cadaeeeb698a4fb
(cherry picked from commit 7aeb8de74e08eb2d305686aa8eff45353973e7d7)
Change-Id: I9c073a21c8257987cf2378012cadaeeeb698a4fb

0 files changed, 0 insertions, 0 deletions