use 'cmd' command for install-multiple

adb install-multiple makes a minimum of 3 calls to the 'pm' command. This
causes at least 3 separate "pm" processes to be spun up. Instead, use the
'cmd' command which runs in the existing system_server process.

Bug: 27483932
Change-Id: Ia1bed405c3d7e675a1a56ff82c692aaa94388a5c