Add unit tests for java.nio.channels.FileChannel open methods

 #open(Path, Set, FIleAttribute …), #open(Path, OpenOption)

Test: cts-tradefed run cts -m CtsLibcoreTestCases -t \
        libcore.java.nio.channels.FileChannelTest

Test: cts-tradefed run cts -m CtsLibcoreTestCases -t \
        libcore.java.nio.file.DefaultFileSystemProvider2Test

Bug: 29031069
Change-Id: Ia4aa1ecb74bec25651efef09c66697e6c6770106