remount: Replace ServiceManager::getService with checkService

Address this build log message:
```
[ 18% 2/11] //system/core/fs_mgr:remount clang++ fs_mgr_remount.cpp
system/core/fs_mgr/fs_mgr_remount.cpp:133:31: warning: 'getService' is deprecated: this polls for 5s, prefer waitForService or checkService [-Wdeprecated-declarations]
        if (auto binder = sm->getService(android::String16("vold"))) {
                              ^
```

Bug: 293695109
Test: adb-remount-test
Change-Id: I3b5c7d338e9307dee58edeb0f6a00ba9a73d46f6