This reverts commit 420ed9ef22efce887900a72964b178cc94274bc3.
Reason for revert: The WeakReference change can prevent memory leak, but
but the extra time spend on creating thousands of WeakReference objects has slowed down system_server startup as reported by b/111658334. Revert this CL for now, will find other approach to prevent memory leak.
Change-Id: I07a5806e36159467459b841f63e2b0c55b6cc10b
6fc752c737