Add tests for java.nio.file.FileSystem & java.nio.file.Paths

Also, removed a part of JavaDoc intended for Windows platform and
removed explicit mention of Unix Platform at couple of other places.

LinuxFileSystem.getMountEntries was fetching mount entry data from
/etc/mtab which is not available on Android, so changed it to /proc/mounts.

Test: cts-tradefed run cts -m CtsLibcoreTestCases -t \
        libcore.java.nio.file.LinuxFileSystemTest
Test: cts-tradefed run cts -m CtsLibcoreTestCases -t \
        libcore.java.nio.file.PathsTest
Bug: 27331465
Bug: 29030792
Change-Id: Ib4c5297138f66604d13a641756d603c9fa82e8f3