Fix race condition bug related to freezing apps.

Consider the following situation:

1. Package is frozen.
2. We try forking the app while frozen, causing a ProcessRecord with
PID 0 to be recorded in mProcessNames. As a result of the failed
fork, removeProcessLocked() tears down that ProcessRecord, but a
special case records it into mRemovedProcesses.
3. Package is unfrozen.
4. We try forking the app, and this time it proceeds normally now
that we're unfrozen.  The new valid ProcessRecord is recorded in
mProcessNames.
5. activityIdleInternalLocked() triggers a clean-up pass of
mRemovedProcesses.  trimApplications() ends up cleaning up the
stale reference from (2) above *by hash key* and not *by reference*,
which causes us to remove the new valid ProcessRecord.  This results
in the valid ProcessRecord in (4) becoming an orphaned PID, which
starts a chain reaction of havoc that ensues.

This issue is fixed by checking the expected ProcessRecord by value
before actually removing it, thus preventing orphaned PIDs.

Test: builds, boots, over 600 installs without orphaned PIDs
Bug: 28395549
Change-Id: I5ea1b31c3fd374ea7f5cc40ff35bb9195d9f3e2b