pm.dexopt.* props in runtime_libart.mk becomes optional

The mk file is designed to provide safe default values which can be
overridden by target-specific mk files. Previously it was difficult to
correctly configure the mk files because the final prop value that
is baked in the system/build.prop is highly dependent (and sensitive as
well) to the mk file inheritance order which is very difficult (and
non-intuitive) to follow.

I9c073a21c8257987cf2378012cadaeeeb698a4fb is an attempt to make it much
easier and intuitive. Specifically, the new `a ?= b` syntax makes the
assignment optional, which means it is used only when there is no
non-optional assignment for the same prop regardless of the relative
ordering among them. In addition, the change prohibits having multiple
non-optional prop assignments for the same prop name.

pm.dex.* prop in runtime_libart.mk are now set using the `a ?= b` syntax
to explicitly mark that they provide default values.

Bug: 117892318
Bug: 158735147
Test: m
Change-Id: I044486d313d699607cd54222ae34d9eae24762b9