[aapt2] Use FeatureFlagsFilter in link command

The behavior of the filter depends on the minSdkVersion. If > UDC,
manifest elements will be untouched. Otherwise, they will be removed
depending on the value of the flag in the `android:featureFlag`
attribute.

Bug: 297373084
Test: atest aapt2_tests
Change-Id: Ib07c45752a4cc00bfdae7fedc745ce267ec7082a
diff --git a/tools/aapt2/cmd/Link.h b/tools/aapt2/cmd/Link.h
index 26713fd..dc18b1c 100644
--- a/tools/aapt2/cmd/Link.h
+++ b/tools/aapt2/cmd/Link.h
@@ -330,7 +330,11 @@
             "should only be used together with the --static-lib flag.",
         &options_.merge_only);
     AddOptionalSwitch("-v", "Enables verbose logging.", &verbose_);
-    AddOptionalFlagList("--feature-flags", "Placeholder, to be implemented.", &feature_flags_args_);
+    AddOptionalFlagList("--feature-flags",
+                        "Specify the values of feature flags. The pairs in the argument\n"
+                        "are separated by ',' and the name is separated from the value by '='.\n"
+                        "Example: \"flag1=true,flag2=false,flag3=\" (flag3 has no given value).",
+                        &feature_flags_args_);
   }
 
   int Action(const std::vector<std::string>& args) override;