adb: mkdirs fixes

Fix pathological case where the directory to be created can't be created
because there is already a file there. This was previously returning
success because the wrong var was passed to directory_exists().

Fix test to exercise this situation. Also clarify tests.

Change-Id: I0dc0f14084e0eda4e1498874d4ab2a6445d322ac
Signed-off-by: Spencer Low <CompareAndSwap@gmail.com>
(cherry picked from commit 85c45bd5)