Refactor mixed build allowlist handling

This refactoring prepares for introduction of bazel prod mode, an
alternative mechanism for mixed builds allowlist handling.

 * Decide bazel-mode as close to soong_build main as possible
 * BazelContext itself decides whether a module is allowlisted
 * Separate bp2build and mixed build allowlist

Test: m nothing, manually verified all modules are mixed build disabled
(via metrics)
Test: USE_BAZEL_ANALYSIS=1 m nothing, manually verified that mixed build
disabled/enabled modules are identical before and after change.

Change-Id: I0f55d8b85000cb4a871a099edc6d7d868d7df509
diff --git a/android/testing.go b/android/testing.go
index b4429ca..ef5e5a9 100644
--- a/android/testing.go
+++ b/android/testing.go
@@ -457,7 +457,7 @@
 
 // RegisterForBazelConversion prepares a test context for bp2build conversion.
 func (ctx *TestContext) RegisterForBazelConversion() {
-	ctx.SetRunningAsBp2build()
+	ctx.config.BuildMode = Bp2build
 	RegisterMutatorsForBazelConversion(ctx.Context, ctx.bp2buildPreArch)
 }