Android emulator builders are soon to join the trybot set. To avoid
flaky runs, work around a longstanding adb bug where concurrent adb
commands sometimes fail.

I haven't seen the problem on actual devices until recently. It seems
that the recently added "adb wait-for-device" can introduce flakyness
with errors such as:

adb: error: failed to get feature set: protocol fault (couldn't read status): Connection reset by peer

Instead of working around that, give up and serialize use of adb
everywhere.

Fixes #23795
Updates #23824

Change-Id: If347c9981fa32ff8a1e14b7454f122ef682450a6
Reviewed-on: https://go-review.googlesource.com/c/163625Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>