Replace #apex with #systemapi

The intended use is
- #apex is APEX-visible symbols provided by apex libraries
- #systemapi is APEX-visible symbols provided by platform libraries

Since libandroid is a platform library, update the annotation to
sytemapi.

Bug: 239274367
Bug: 265485106
Test: TH
Change-Id: I28db5cd0d3a76afa1a7ecac042ee02dd74faa2f1

native/android/libandroid.map.txt

 LIBANDROID {
   global:
-    AActivityManager_addUidImportanceListener; # apex # introduced=31
-    AActivityManager_removeUidImportanceListener; # apex # introduced=31
-    AActivityManager_isUidActive; # apex # introduced=31
-    AActivityManager_getUidImportance; # apex # introduced=31
+    AActivityManager_addUidImportanceListener; # systemapi # introduced=31
+    AActivityManager_removeUidImportanceListener; # systemapi # introduced=31
+    AActivityManager_isUidActive; # systemapi # introduced=31
+    AActivityManager_getUidImportance; # systemapi # introduced=31
     AAssetDir_close;
     AAssetDir_getNextFileName;
     AAssetDir_rewind;