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 7aeb8de74e)
Change-Id: I9c073a21c8257987cf2378012cadaeeeb698a4fb
