adb: don't use setenv after forking.

Previously, for `adb shell`, we were using setenv after forking to set
up the child's environment. This would occasionally deadlock in the
child, which would cause the main thread to deadlock waiting for the
child to complete. This patch constructs the environment before forking
and passes it to execle, eliminating the deadlock.

Bug: http://b/25847115
Change-Id: I720d472770564b1449819ddaab945a89844244a8