Accept --max-fps before Android 10

KEY_MAX_FPS_TO_ENCODER existed privately before Android 10:
<#488 (comment)>

README.md

@@ -137,12 +137,14 @@ scrcpy -b 2M  # short version
 
 #### Limit frame rate
 
-On devices with Android >= 10, the capture frame rate can be limited:
+The capture frame rate can be limited:
 
 ```bash
 scrcpy --max-fps 15
 ```
 
+This is officially supported since Android 10, but may work on earlier versions.
+
 #### Crop
 
 The device screen may be cropped to mirror only part of the screen.

app/scrcpy.1

@@ -43,7 +43,7 @@ Print this help.
 
 .TP
 .BI "\-\-max\-fps " value
-Limit the framerate of screen capture (only supported on devices with Android >= 10).
+Limit the framerate of screen capture (officially supported since Android 10, but may work on earlier versions).
 
 .TP
 .BI "\-m, \-\-max\-size " value

app/src/cli.c

@@ -42,8 +42,8 @@ scrcpy_print_usage(const char *arg0) {
         "        Print this help.\n"
         "\n"
         "    --max-fps value\n"
-        "        Limit the frame rate of screen capture (only supported on\n"
-        "        devices with Android >= 10).\n"
+        "        Limit the frame rate of screen capture (officially supported\n"
+        "        since Android >= 10, but may work on earlier versions).\n"
         "\n"
         "    -m, --max-size value\n"
         "        Limit both the width and height of the video to value. The\n"

server/src/main/java/com/genymobile/scrcpy/ScreenEncoder.java

@@ -19,6 +19,7 @@ public class ScreenEncoder implements Device.RotationListener {
 
     private static final int DEFAULT_I_FRAME_INTERVAL = 10; // seconds
     private static final int REPEAT_FRAME_DELAY_US = 100_000; // repeat after 100ms
+    private static final String KEY_MAX_FPS_TO_ENCODER = "max-fps-to-encoder";
 
     private static final int NO_PTS = -1;
 
@@ -150,11 +151,10 @@ private static MediaFormat createFormat(int bitRate, int maxFps, int iFrameInter
         // display the very first frame, and recover from bad quality when no new frames
         format.setLong(MediaFormat.KEY_REPEAT_PREVIOUS_FRAME_AFTER, REPEAT_FRAME_DELAY_US); // µs
         if (maxFps > 0) {
-            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q) {
-                format.setFloat(MediaFormat.KEY_MAX_FPS_TO_ENCODER, maxFps);
-            } else {
-                Ln.w("Max FPS is only supported since Android 10, the option has been ignored");
-            }
+            // The key existed privately before Android 10:
+            // <https://android.googlesource.com/platform/frameworks/base/+/625f0aad9f7a259b6881006ad8710adce57d1384%5E%21/>
+            // <https://github.com/Genymobile/scrcpy/issues/488#issuecomment-567321437>
+            format.setFloat(KEY_MAX_FPS_TO_ENCODER, maxFps);
         }
         return format;
     }