Use user id 0 to get missing appId for debugger

When forceStopPackageLocked is used to kill existing app process for
relaunching with debugger, user id could be -1 and getPackageUid will
already return -1. This results in killPackageProcessesLocked() not
able to find and kill the app, and wait for debugger will.

This also revert commit 55c9d739 (ag/969408) which tried to fix the
same problem but got it wrong. It's valid to -D when the process is
already running, as we're supposed to kill it. The really problem is
that the killing fails because of bad appId.

bug: 28980761
Change-Id: Ibebdcc26865464216a1f4b6773e540921cdad7d2