Detect incompatible Android versoin (API < 21)

[isotopeneo]

Initial title: Segmentation fault when trying scrcpy on Kindles

/usr/local/share/scrcpy/scrcpy-server.jar: 1 file pushed. 1.8 MB/s (17874 bytes in 0.010s)
error: closed
2018-09-26 01:12:43.306 scrcpy[2646:20517898] ERROR: "adb reverse" returned with value 1
2018-09-26 01:12:43.306 scrcpy[2646:20517898] WARN: 'adb reverse' failed, fallback to 'adb forward'
Segmentation fault

Model: Kindle Fire HDX 8.9 (3rd generation)

[rom1v]

scrcpy requires Android >= 5.

The segfault occurs on the device.

See #174.

[eku]

@rom1v Since this issue occurs again and again, is it possible that the CLI program tests the Android version and gives a hint?

[rom1v]

The version could be easily detected on the server-side, unfortunately the crash occurs before the server code is actually executed.

It is possible to check on the client with adb shell getprop ro.build.version.release. I didn't want to do that because I thought getprop was written in Java (like settings), which would require to exec app_process (which would delay the start of the client by 500ms or 1s). But in fact, it seems it's native, and fast enough:

$ time adb shell getprop ro.build.version.release
4.3

real	0m0,028s
user	0m0,000s
sys	0m0,005s

Since adb reverse exists only since 5.0, we can add a check only if adb reverse fails (before adb forward fallback).