Add PRODUCT_BROKEN_VERIFY_USES_LIBRARIES.

If set to true, it disables <uses-library> checks for a product. It is
possible to override with environment variable RELAX_USES_LIBRARY_CHECK
on the command-line.

Per product configuration is needed rather than per board configuration
(such as BUILD_BROKEN_* variables) because of the specifics of
<uses-library> checks: they depend on the product packages list, and
not on the board config.

One example of a product family that needs to disable the checks are
the SDK builds (e.g. sdk_gphone_x86_64), see b/183339664.

Bug: 183339664
Bug: 132357300
Test: treehugger
Change-Id: Ia58559e4e70163da94e9eb7b8a6133e0cbecd459