adb: move all cleanup to a function with defined ordering.

We want to explicitly define the order in which we teardown adb, so
move all of the at_quick_exits sprinkled throughout into one function
containing all of the cleanup functions.

Bug: http://b/37104408
Test: adb kill-server; adb start-server
Change-Id: I394f5782eb147e394d4b87df1ba364c061de4b90