Make Lint FlaggedApi violations errors

Change Lint FlaggedApis from warnings to errors.

Bug: 303434307
Test: m lint-check -k
Flag: EXEMPT not possible to flag baseline changes
Change-Id: I990ce57cd5cdac44ee8b93565b45aa15790ad697
diff --git a/java/lint_defaults.txt b/java/lint_defaults.txt
index b8ce95c..262e184 100644
--- a/java/lint_defaults.txt
+++ b/java/lint_defaults.txt
@@ -123,9 +123,6 @@
 --warning_check SupportAnnotationUsage
 --warning_check UniqueConstants
 --warning_check UseSdkSuppress
-# TODO(b/303434307) The intent is for this to be set to error severity
-# once existing violations are cleaned up
---warning_check FlaggedApi
 
 --warning_check ExactAlarm
 --warning_check ExpiredTargetSdkVersion