adb: fix stat on Windows.

stat on Windows fails with ENOENT when passed a path with a trailing
slash or backslash, regardless of whether the target is actually a
directory. Emulate the correct POSIX behavior by stripping trailing
path separators and then checking if the target is a directory if
successful.

Bug: http://b/30481559
Bug: https://code.google.com/p/android/issues/detail?id=214633
Change-Id: I1d398d19a9bce1ecb3fdc4aabc31aa98c82c3f93
Test: Relevant adb_tests pass on Linux and Windows 10.