Android: implement headphone detection thus enabling pause on unplug (FS#12097).

Listen to headphone plug events. There are currently two glitches with this:
- Android takes a while until it reports the unplug event, so there will be
  some delay until playback gets paused. This is an Android limitation.
- Rockbox debounces headphone state changes for one second. Therefore playback
  will shortly be routed to the speaker on unplug until Rockbox does the actual
  pause.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@29956 a1c6a512-1295-4272-9138-f99709370657

firmware/target/hosted/android/button-android.c

@@ -31,6 +31,10 @@
 #include "powermgmt.h"
 
 extern JNIEnv *env_ptr;
+extern jclass  RockboxService_class;
+extern jobject RockboxService_instance;
+
+static jfieldID _headphone_state;
 static int last_y, last_x;
 static int last_btns;
 
@@ -110,6 +114,20 @@ Java_org_rockbox_RockboxFramebuffer_buttonHandler(JNIEnv*env, jclass class,
 
 void button_init_device(void)
 {
+    jmethodID initHeadphoneMonitor = (*env_ptr)->GetMethodID(env_ptr,
+                                                           RockboxService_class,
+                                                           "initHeadphoneMonitor",
+                                                           "()V");
+    /* start the monitor */
+    (*env_ptr)->CallVoidMethod(env_ptr,
+                               RockboxService_instance,
+                               initHeadphoneMonitor);
+
+    /* cache the headphone state field id */
+    _headphone_state = (*env_ptr)->GetFieldID(env_ptr,
+                                            RockboxService_class,
+                                            "headphone_state",
+                                            "I");
 }
 
 int button_read_device(int *data)
@@ -127,3 +145,13 @@ int button_read_device(int *data)
 
     return btn;
 }
+
+
+/* Tell if anything is in the jack. */
+bool headphones_inserted(void)
+{
+    int state = (*env_ptr)->GetIntField(env_ptr, RockboxService_instance, _headphone_state);
+    /* 0 is disconnected, 1 and 2 are connected */
+    return (state == 0) ? false : true;
+}
+