ActivityManager: process stop needs a process to stop

Make process a required argument for am process stop to avoid silent
failure.

Minor usage clean-up for am process commands.

Bug: 129892812
Test: manual

(cherry picked from commit 4e6a314e)

Change-Id: Ia79e3c7e8b4cbc3eb6ab548b5cd0f61306875fb0
Merged-In: Ib5f3c225c79af65f66c22b24abbef210bb383c8e