303434307: Enable flagged API checking

Also downgrade the new UseSdkSuppress check from error to warning.
And temporarily downgrades FlaggedApi from error to warning until
existing violations are resolved.

Test: The build
Bug: 303434307
Change-Id: Id7802621d567815470dee1aeb815abe214f098cd
diff --git a/java/lint.go b/java/lint.go
index 34720e5..0b3baf8 100644
--- a/java/lint.go
+++ b/java/lint.go
@@ -491,6 +491,7 @@
 
 	cmd.BuiltTool("lint").ImplicitTool(ctx.Config().HostJavaToolPath(ctx, "lint.jar")).
 		Flag("--quiet").
+		Flag("--include-aosp-issues").
 		FlagWithInput("--project ", lintPaths.projectXML).
 		FlagWithInput("--config ", lintPaths.configXML).
 		FlagWithOutput("--html ", html).