snapuserd: Remove dependence on libfs_mgr.

The only function we use from libfs_mgr is WaitForFile, so factor that
out into libfs_mgr_file_wait. libfs_mgr has too much dependence on AOSP
internals to build on the host.

Bug: 288273605
Test: builds
Change-Id: I8e5eb4075ae01b9ca64074422171a487179e7943
diff --git a/fs_mgr/Android.bp b/fs_mgr/Android.bp
index 4e4d20e..87db98b 100644
--- a/fs_mgr/Android.bp
+++ b/fs_mgr/Android.bp
@@ -171,6 +171,25 @@
     ],
 }
 
+cc_library_static {
+    name: "libfs_mgr_file_wait",
+    defaults: ["fs_mgr_defaults"],
+    export_include_dirs: ["include"],
+    cflags: [
+        "-D_FILE_OFFSET_BITS=64",
+    ],
+    srcs: [
+        "file_wait.cpp",
+    ],
+    shared_libs: [
+        "libbase",
+    ],
+    host_supported: true,
+    ramdisk_available: true,
+    vendor_ramdisk_available: true,
+    recovery_available: true,
+}
+
 cc_binary {
     name: "remount",
     defaults: ["fs_mgr_defaults"],