Introduce BazelStringOrLabelFromProp.

Soong supports string properties, but they are overloaded, and can mean
one of three things:

* path reference
* module reference
* string literal

Bazel has different types: label and string attributes. Thus there needs
to be a way to categorize them correctly in bp2build.

This CL introduces a new function to be used on properties like
apex_key.private_key / apex_key.public_key, as well as
android_app.certificate / apex.certificate.

It is important to disambiguate the prop betenn a string literal
attribute or file/rule target label attribute, so this functions does
just that.  The new attributes are then further handled by their
respective macros (apex_key, android_binary, apex).

Bug: 253557437
Fixes: 253557437
Test: presubmits, new tests

Change-Id: Id8111cdd60d3aabcae7d17fe9da84d0ee3966023