fs_mgr_overlayfs: Cleanup -user build stubs

* Categorize functions in fs_mgr_overlayfs.h into three classes:
  - Type 1: common and non-critical utilities.
  - Type 2: internal routines for facilitating remount.
  - Type 3: external entry points for users of fs_mgr_overlayfs, like
            fs_mgr_overlayfs_mount_all().
* Move type 1 to common utils header fs_mgr_priv.h & fs_mgr.cpp.
* Move type 2 to new private header fs_mgr_priv_overlayfs.h.
* Keep type 3 in fs_mgr_overlayfs.h.
* Move set-verity-state.cpp under fs_mgr so it can include
  fs_mgr_priv_overlayfs.h. File is reformatted as a result. We should
  eventually merge and dedup set-verity-state and fs_mgr_remount.
* Add myself to OWNERS for remount-related maintenance work.

Bug: 241179247
Bug: 241688845
Test: Full build -user and -userdebug build.
Test: Presubmit
Test: Treehugger run v2/android-gki/adb_remount
Change-Id: Id5fd0e2b12c693939d712a586dd553cc4d8bfeb1
diff --git a/fs_mgr/Android.bp b/fs_mgr/Android.bp
index 49761ac..45cd3c0 100644
--- a/fs_mgr/Android.bp
+++ b/fs_mgr/Android.bp
@@ -254,3 +254,39 @@
         "clean_scratch_files",
     ],
 }
+
+cc_binary {
+    name: "set-verity-state",
+    srcs: ["set-verity-state.cpp"],
+    shared_libs: [
+        "libbase",
+        "libbinder",
+        "libcrypto",
+        "libcrypto_utils",
+        "libfs_mgr_binder",
+        "libutils",
+    ],
+    static_libs: [
+        "libavb_user",
+    ],
+    header_libs: [
+        "libcutils_headers",
+    ],
+
+    cflags: ["-Werror"],
+    cppflags: [
+        "-DALLOW_DISABLE_VERITY=0",
+    ],
+    product_variables: {
+        debuggable: {
+            cppflags: [
+                "-UALLOW_DISABLE_VERITY",
+                "-DALLOW_DISABLE_VERITY=1",
+            ],
+        },
+    },
+    symlinks: [
+        "enable-verity",
+        "disable-verity",
+    ],
+}