bp2build support for stem

By default, the artifacts generated by cc_binary and cc_library in Soong
track the module name. But Soong supports overidding this using the stem
property. e.g.
```
cc_library {
  name: "foo.1.2",
  stem: "foo",
}
```
will generate foo.so

This CL adds this property to bp2build

Test: bp2build unit tests
Bug: 240563612
Change-Id: I1dfed870d5bad450511b72c397d2355c01fa3b60