Address old review comments in adb_auth_host.cpp.

Comments from:
https://android-review.googlesource.com/#/c/212781/4/adb/adb_auth_host.cpp@107

Bug: http://b/28152031
Change-Id: I27d062f3eeb8db90f94b1b4f5d7204000a7ac73d
Test: manually removed ~/.android/adb* and checked they were recreated correctly.