Capture first display for multi-display devices

When no display id is explicitly specified, the first display is
captured, and a warning message is printed out. This is to make
the utility to be backward compatible with previous versions so
tools depending on it continue to work.

Bug: 241285477
Test: manual, run screencap command to verify
Change-Id: I713e1e5c625fd80ecb5587e0b2313f58baaffd40

cmds/screencap/screencap.cpp

@@ -159,12 +159,14 @@ int main(int argc, char** argv)
     }
 
     if (!displayId) { // no diplsay id is specified
-        if (ids.size() == 1) {
-            displayId = ids.front();
-        } else {
-            fprintf(stderr, "Please specify a display ID (-d display-id) for multi-display device.\n");
+        displayId = ids.front();
+        if (ids.size() > 1) {
+            fprintf(stderr,
+                    "[Warning] Multiple displays were found, but no display id was specified! "
+                    "Defaulting to the first display found, however this default is not guaranteed "
+                    "to be consistent across captures. A display id should be specified.\n");
+            fprintf(stderr, "A display ID can be specified with the [-d display-id] option.\n");
             fprintf(stderr, "See \"dumpsys SurfaceFlinger --display-id\" for valid display IDs.\n");
-            return 1;
         }
     }