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
diff --git a/libs/input/KeyCharacterMap.cpp b/libs/input/KeyCharacterMap.cpp
index d571917..a4cd239 100644
--- a/libs/input/KeyCharacterMap.cpp
+++ b/libs/input/KeyCharacterMap.cpp
@@ -1247,7 +1247,7 @@
     }
 
     // Ensure that we consumed the entire token.
-    if (mTokenizer->nextToken(WHITESPACE).isEmpty()) {
+    if (mTokenizer->nextToken(WHITESPACE).empty()) {
         return NO_ERROR;
     }