Use $ANDROID_SERIAL if no selector is specified

Like adb, read the ANDROID_SERIAL environment variable to select a
device by serial if no explicit selection (-s, -d, -e or --tcpip=<addr>)
is provided via the command line.

Fixes #3111 <https://github.com/Genymobile/scrcpy/issues/3111>
PR #3113 <https://github.com/Genymobile/scrcpy/pull/3113>