fs_mgr: add remount command

There is currently no good option for callers to setup overlayfs
on-device, it is automated as part of the adb services.  Add a
remount command that does what is needed that simulates the salient
behaviors of the adb remount command.

Clean up some noise restoring device to original state when done.

Test: adb-remount-test.sh
Bug: 122602260
Change-Id: Idf213800a8182cb1c51600c8f574df8a8cd68d4a
diff --git a/fs_mgr/Android.bp b/fs_mgr/Android.bp
index 974e13e..a476cd8 100644
--- a/fs_mgr/Android.bp
+++ b/fs_mgr/Android.bp
@@ -111,3 +111,26 @@
         "libgsi_headers",
     ],
 }
+
+cc_binary {
+    name: "remount",
+    defaults: ["fs_mgr_defaults"],
+    shared_libs: [
+        "libbase",
+        "libfs_mgr",
+    ],
+    srcs: [
+        "fs_mgr_remount.cpp",
+    ],
+    cppflags: [
+        "-DALLOW_ADBD_DISABLE_VERITY=0",
+    ],
+    product_variables: {
+        debuggable: {
+            cppflags: [
+                "-UALLOW_ADBD_DISABLE_VERITY",
+                "-DALLOW_ADBD_DISABLE_VERITY=1",
+            ],
+        },
+    },
+}