Fix -XepDisableAllWarnings flag when passed on its own

Without this change, the flag would not work when passed as the single Error Prone argument.  Not sure how to add a regression test for this, but the fix works locally.

Fixes #3462

FUTURE_COPYBARA_INTEGRATE_REVIEW=#3462 from msridhar:fix-disable-all-warnings 89f2c1d
PiperOrigin-RevId: 481263162

check_api/src/main/java/com/google/errorprone/scanner/ScannerSupplier.java

@@ -143,7 +143,8 @@ public ScannerSupplier applyOverrides(ErrorProneOptions errorProneOptions) {
         && errorProneOptions.getFlags().isEmpty()
         && !errorProneOptions.isEnableAllChecksAsWarnings()
         && !errorProneOptions.isDropErrorsToWarnings()
-        && !errorProneOptions.isDisableAllChecks()) {
+        && !errorProneOptions.isDisableAllChecks()
+        && !errorProneOptions.isDisableAllWarnings()) {
       return this;
     }