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/media/libdatasource/DataSourceFactory.cpp b/media/libdatasource/DataSourceFactory.cpp
index 81a6cd0..e9936a3 100644
--- a/media/libdatasource/DataSourceFactory.cpp
+++ b/media/libdatasource/DataSourceFactory.cpp
@@ -92,7 +92,7 @@
 
         source = NuCachedSource2::Create(
                 mediaHTTP,
-                cacheConfig.isEmpty() ? NULL : cacheConfig.c_str(),
+                cacheConfig.empty() ? NULL : cacheConfig.c_str(),
                 disconnectAtHighwatermark);
     } else if (!strncasecmp("data:", uri, 5)) {
         source = DataURISource::Create(uri);