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/libnblog/PerformanceAnalysis.cpp b/media/libnblog/PerformanceAnalysis.cpp
index 507295a..7982805 100644
--- a/media/libnblog/PerformanceAnalysis.cpp
+++ b/media/libnblog/PerformanceAnalysis.cpp
@@ -393,7 +393,7 @@
             PerformanceAnalysis& curr = hash.second;
             // write performance data to console
             curr.reportPerformance(&body, thread.first, hash.first);
-            if (!body.isEmpty()) {
+            if (!body.empty()) {
                 dumpLine(fd, indent, body);
                 body.clear();
             }
diff --git a/media/libnblog/Reader.cpp b/media/libnblog/Reader.cpp
index 36e4da5..26d12cd 100644
--- a/media/libnblog/Reader.cpp
+++ b/media/libnblog/Reader.cpp
@@ -279,7 +279,7 @@
         default:
             break;
         }
-        if (!body.isEmpty()) {
+        if (!body.empty()) {
             dprintf(fd, "%.*s%s %s\n", (int)indent, "", timestamp.c_str(), body.c_str());
             body.clear();
         }