Validate the ProcessRecord while iterating LRU list with ProcLock

Fix a race condition where the ProcessRecord has been invalidated
but it hasn't been removed from LRU list, meanwhile another thread
iterates the LRU list with the ProcLock, thus the ProcessRecord
should be validated.

Bug: 180849723
Test: atest CtsAppTestCases:ActivityManagerTest
Test: Manual - randomly kill other processes during above test
Change-Id: I76570a7717b98b499b865acf590af7a2133c36d9