Allow individual projects to enforce a property's value

Allow individual projects to enforce a property's value

Some projects require system properties to be set to a specific
value (for example, a shared library needing a property pointing
to its own path) in order to work correctly, but some device
configurations are mistakenly setting those properties with the
wrong value (usually inherited from the original OEM build).

"PRODUCT_PROPERTY_UBER_OVERRIDES += property=value" can (and
should) be used in that project's makefile to ensure the value
is the correct one. This variable is intended for software projects,
and should never be used in product makefiles (BoardConfig, cm.mk,
AndroidProduct)

Change-Id: I1986e7c444e51cce8b198e43fdc793fad16d6276