Ports are not reserved by Android (or by any other OS) but are assigned by IANA for specific use. If a port is free i.e. no process is connected to or listening on that, you may use that with your program. But this may break the app's functionality for which the port is reserved. Otherwise you'll have to modify the source code and rebuild all the programs that are using a fixed port. It doesn't seem logical and practically possible.
For instance UDP port 68 is reserved for DHCP clients and the same is on Android:
# ss -up
Recv-Q Send-Q Local Address:Port Peer Address:Port
0 0 192.168.1.11:68 192.168.1.1:67 users:(("system_server",pid=4521,fd=249))
So if you want to use this port for your program, your will have to modify Android source code to replace all instances of port 68 with some other unused port and then rebuild the ROM.
However you are free to use it when device is not connected to some network through WiFi or mobile data which obviously doesn't make much sense. Or you can configure static IP, Gateway and DNS so that DHCP is no more needed and the port gets free for other usage.
Further reading: 1 2 3