Downgrade UnspecifiedImmutableFlag errors to warnings

There are preexisting occurances of it in the source tree.

Bug: 247885568
Test: Presubmits
Change-Id: Ieac1e297414e6abb91460af5da30461d20e7a6ab
diff --git a/java/lint_defaults.txt b/java/lint_defaults.txt
index 01e7e6e..bff7221 100644
--- a/java/lint_defaults.txt
+++ b/java/lint_defaults.txt
@@ -91,6 +91,7 @@
 --warning_check StringFormatInvalid                # 148 occurences in 11 modules
 --warning_check StringFormatMatches                # 4800 occurences in 30 modules
 --warning_check UnknownId                          # 8 occurences in 7 modules
+--warning_check UnspecifiedImmutableFlag
 --warning_check ValidFragment                      # 12 occurences in 5 modules
 --warning_check ValidRestrictions                  # 5 occurences in 1 modules
 --warning_check WebViewLayout                      # 3 occurences in 1 modules