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

android/defaults.go

@@ -448,7 +448,7 @@ func (defaultable *DefaultableModuleBase) applyDefaults(ctx TopDownMutatorContex
 	}
 
 	for _, defaults := range defaultsList {
-		if ctx.Config().runningAsBp2Build {
+		if ctx.Config().BuildMode == Bp2build {
 			applyNamespacedVariableDefaults(defaults, ctx)
 		}