Use command instead of which to locate/run adb

If adb is not found on PATH, which will silently fail and evaluate to
the empty string. This will cause the remaining arguments to be
interpreted as a command to run in the shell, which is generally
undesirable. (Consider, for example, "adb reboot" -> "reboot".)

Bug: 24473359
Test: Just run adb before lunch so it's not on PATH

Change-Id: I6b20722add6c67d1d2627f963dc66095502ab816
Signed-off-by: Saagar Jha <saagarjha@google.com>