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