Add android::fs_mgr namespace for new Fstab code

Should have been done a while ago, but better late than never.

Test: treehugger
Change-Id: I0ea6e8d459cd3f3b3ce2d00a7a6a9786d52c52dd
diff --git a/init/ueventd.cpp b/init/ueventd.cpp
index 7545d53..399ea4c 100644
--- a/init/ueventd.cpp
+++ b/init/ueventd.cpp
@@ -245,7 +245,7 @@
     uevent_handlers.emplace_back(std::make_unique<DeviceHandler>(
             std::move(ueventd_configuration.dev_permissions),
             std::move(ueventd_configuration.sysfs_permissions),
-            std::move(ueventd_configuration.subsystems), fs_mgr_get_boot_devices(), true));
+            std::move(ueventd_configuration.subsystems), android::fs_mgr::GetBootDevices(), true));
     uevent_handlers.emplace_back(std::make_unique<FirmwareHandler>(
             std::move(ueventd_configuration.firmware_directories)));