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/libaudioclient/tests/audiotrack_tests.cpp b/media/libaudioclient/tests/audiotrack_tests.cpp
index 8daba0a..2b68225 100644
--- a/media/libaudioclient/tests/audiotrack_tests.cpp
+++ b/media/libaudioclient/tests/audiotrack_tests.cpp
@@ -144,7 +144,7 @@
     EXPECT_EQ(cb->mDeviceId, ap->getAudioTrackHandle()->getRoutedDeviceId());
     String8 keys;
     keys = ap->getAudioTrackHandle()->getParameters(keys);
-    if (!keys.isEmpty()) {
+    if (!keys.empty()) {
         std::cerr << "track parameters :: " << keys << std::endl;
     }
     EXPECT_TRUE(checkPatchPlayback(cb->mAudioIo, cb->mDeviceId));