ActivityManagerService: stay away from zombie content providers

When the lowmemorykiller kills a content provider in an unfortunate
moment, an innocent client app can become attached to a zombie provider
and be unconditionally killed moments after, when the dead provider is
being removed.
Prevent this race by addition of isAlive function to Process.java that
determines whether a process is still running and is not only a zombie,
add a check of the state of the provider process to getContentProviderImpl.

Change-Id: Id753b8dab103e4b447c4138468938ca9f5cb857d
ContentProviderRecord: Remove external process reference when finalized

ContentProviderRecords is removed from provider map by activity manager service
when the process hosting the content providers terminates and this causes GC
to free the content provider records but the reference to external process
using the content provider isn't cleared. Fix is to clear the reference and
unlink for death notification.

Change-Id: I67fd10cee57b02257792d0c9b4569fe11dea7648