Warn when defining a 32-bit product on a 64-bit device.

Building a 32-bit-app-only product on a 64-bit capable device is likely
to be unintentional. Print a warning if this is done implicitly,
e.g. via forgetting to define the TARGET_SUPPORTS_64_BIT_APPS variable.

Also change existing references to only treat 'true' as a positive
value, rather than all non-empty ones.

Bug: 110918674
Test: lunched 64-bit-device product that does and doesn't set this var
Change-Id: I021157c94ec3ac1c690547c8992768e58c89c287