binder: don't unlock procs while scanning contexts

Releasing the procs lock while freezing a binder context allows for
other processes to modify the process list while the scan is still
ongoing.

Don't release the process locks during the scan operatoin, but store
matching processes in a dynamic array and process them at a later phase.

Signed-off-by: Marco Ballesio <balejs@google.com>
Bug: 176996063
Test: verified that all contexts are correctly frozen and unfrozen
Change-Id: Iea527e3b9188b04303f8b9b08b404e0c062a0189