Migrate from android::String isEmpty to empty

This empty method is different from the old one - it aligns with
std::string definition.

Bug: 295394788
Test: make checkbuild
Change-Id: I890b3dfa930002f4406e9d9aec024f94ada1cad0
diff --git a/services/mediaresourcemanager/ResourceObserverService.cpp b/services/mediaresourcemanager/ResourceObserverService.cpp
index 33525fd..6c5cecf 100644
--- a/services/mediaresourcemanager/ResourceObserverService.cpp
+++ b/services/mediaresourcemanager/ResourceObserverService.cpp
@@ -138,7 +138,7 @@
                 String8 enabledEventsStr;
                 for (auto &event : sEvents) {
                     if (((uint64_t)observable.eventFilter & (uint64_t)event) != 0) {
-                        if (!enabledEventsStr.isEmpty()) {
+                        if (!enabledEventsStr.empty()) {
                             enabledEventsStr.append("|");
                         }
                         enabledEventsStr.append(toString(event).c_str());