bp2build: add allowlist for package-level conversions.

This CL adds the support for specifying lists of directories in
build/soong/android/bazel.go, which are then written into
out/soong/bp2build/MANIFEST. Using this configuration,
modules/directories can either default to bp2build_available: true or
false, while still retaining the ability to opt-in or out at the module level.

It also ensures that ConvertWithBp2Build returns true iff the module
type has a registered bp2build converter.

Test: go tests
Test: demo.sh full
Test: TreeHugger presubmits for bp2build and mixed builds.

Change-Id: I0e0f6f4b1b2ec045f2f1c338f7084defc5d23a55

android/filegroup.go

@@ -53,7 +53,7 @@ func (bfg *bazelFilegroup) GenerateAndroidBuildActions(ctx ModuleContext) {}
 
 func FilegroupBp2Build(ctx TopDownMutatorContext) {
 	fg, ok := ctx.Module().(*fileGroup)
-	if !ok || !fg.ConvertWithBp2build() {
+	if !ok || !fg.ConvertWithBp2build(ctx) {
 		return
 	}