Add function to read an adb property

This will allow to read the property "service.adb.tcp.port" to know if
the TCP/IP mode is enabled on the device, and which listening port is
used.

PR #2827 <https://github.com/Genymobile/scrcpy/pull/2827>

app/src/adb.h

@@ -65,6 +65,13 @@ adb_connect(struct sc_intr *intr, const char *ip_port, unsigned flags);
 bool
 adb_disconnect(struct sc_intr *intr, const char *ip_port, unsigned flags);
 
+/**
+ * Execute `adb getprop <prop>`
+ */
+char *
+adb_getprop(struct sc_intr *intr, const char *serial, const char *prop,
+            unsigned flags);
+
 /**
  * Execute `adb get-serialno`
  *