Subsume INTEGRATED_BP2BUILD into other env modes

This refactors bazel-build mode determination logic in soong_ui so it's
clearer which of three possible modes are being used in a given
invocation (NO_BAZEL, GENERATE_BUILD_FILES, or MIXED_BUILDS).

Test: bootstrap tests
Change-Id: I41d2baebf8d560c2cc42db8daa8b936101d453e3

ui/build/build.go

@@ -258,8 +258,8 @@ func Build(ctx Context, config Config, what int) {
 		// Run Soong
 		runSoong(ctx, config)
 
-		if config.Environment().IsEnvTrue("GENERATE_BAZEL_FILES") {
-			// Return early, if we're using Soong as the bp2build converter.
+		if config.bazelBuildMode() == generateBuildFiles {
+			// Return early, if we're using Soong as solely the generator of BUILD files.
 			return
 		}
 	}