runtest: introduce --user parameter

Currently runtest script installs and then runs test apks with default
"adb install" and "adb shell am instrument" behavior.  They are actually
different,  "adb install" uses USER_ALL, "am instrument" uses
USER_CURRENT.  This cl introduces a way to specify exactly which user a
test should run with in a multiple user environment.  This also provides
a work around for b/22976637 for now.

Bug: 19913735
Bug: 22976637
Change-Id: I65e2b2c79768734a8bc9261f8c4693c9f89016b1
3 files changed