bp2build support for cc_prebuilt_library

while cc_prebuilt_library_(shared|static) were already supported,
the raw cc_prebuilt_library module type was not supported by
bp2build. This CL provides that functionality.

Also did a little bit of refactoring and cleaning.

Bug: 225938765
Test: Confirm output BUILD.bazel files look as expected. Confirm
      that build succeeds.
Change-Id: Ic004f88febc0e5b527f2ef420dbd96ecc55d2e09

cc/cc.go

@@ -3556,13 +3556,14 @@ func (c *Module) ConvertWithBp2build(ctx android.TopDownMutatorContext) {
 	case fullLibrary:
 		if !prebuilt {
 			libraryBp2Build(ctx, c)
+		} else {
+			prebuiltLibraryBp2Build(ctx, c)
 		}
 	case headerLibrary:
 		libraryHeadersBp2Build(ctx, c)
 	case staticLibrary:
-
 		if prebuilt {
-			prebuiltLibraryStaticBp2Build(ctx, c)
+			prebuiltLibraryStaticBp2Build(ctx, c, false)
 		} else {
 			sharedOrStaticLibraryBp2Build(ctx, c, true)
 		}