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: Id74502de0e51182aa9629db8dc70739be4483d12
Merged-In: Id74502de0e51182aa9629db8dc70739be4483d12
diff --git a/cmds/atrace/atrace.cpp b/cmds/atrace/atrace.cpp
index 8105626..ab7ef16 100644
--- a/cmds/atrace/atrace.cpp
+++ b/cmds/atrace/atrace.cpp
@@ -796,7 +796,7 @@
     bool ok = true;
     while (!tokenizer->isEol()) {
         String8 token = tokenizer->nextToken(" ");
-        if (token.isEmpty()) {
+        if (token.empty()) {
             tokenizer->skipDelimiters(" ");
             continue;
         }