Migrate from android::String isEmpty to empty [aapt]

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

Bug: 295394788
Test: make checkbuild
Change-Id: I1900f2285ee3c3b1eb57d2cef3990896a64dae01
diff --git a/tools/aapt/SourcePos.cpp b/tools/aapt/SourcePos.cpp
index e130286..354a65c 100644
--- a/tools/aapt/SourcePos.cpp
+++ b/tools/aapt/SourcePos.cpp
@@ -78,7 +78,7 @@
         break;
     }
     
-    if (!this->file.isEmpty()) {
+    if (!this->file.empty()) {
         if (this->line >= 0) {
             fprintf(to, "%s:%d: %s%s\n", this->file.c_str(), this->line, type, this->error.c_str());
         } else {