improve write timeout handling

Return type of write() method changed to void. The return value was redundant before, as it always was the request length or an exception was thrown.

If timeout is reached, write() now throws a SerialTimeoutException with ex.bytesTransferred filled with known transferred bytes.

Added CommonUsbSerialPort.getReadEndpoint() and .getWriteEndpoint() to assist in setting the optimal write buffer size with port.setWriteBufferSize(port.getWriteEndpoint().getMaxPacketSize()).

By default the write buffer size is > MaxPacketSize and the Linux kernel splits writes in chunks. When the timeout occurs, it's unknown how many chunks have already been transferred and the exception typically stores 0. With optimal write buffer size, this value is known and stored in SerialTimeoutException, but due to more kernel round trips write() might take slightly longer().

build.gradle

@@ -6,7 +6,7 @@ buildscript {
         google()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:4.1.1'
+        classpath 'com.android.tools.build:gradle:4.1.2'
     }
 }