Introduce product variable PRODUCT_HIDDEN_API_EXPORTABLE_STUBS

The new product variable behaves identical to the build flag
RELEASE_HIDDEN_API_EXPORTABLE_STUBS: if the variable is set to true, the
hiddenapi flags are generated from the "exportable" stubs (i.e. the
stubs that does not include READ_WRITE permission or DISABLE state
flagged apis) instead of the "everything" stubs (i.e. the stubs that
includes all flagged apis regardless of the state of the flags). If the
variable is set to false, the stubs are generated from the "everything"
stubs.

This product variable will be utilized for `sdk_with_runtime_apis`
product in the future changes.

Test: m nothing
Bug: 326310637
Change-Id: I8872d0b0617eb0f25da32611837ad823c77b0df7