media: improve ImageReader/Writer native memory management

* Hook up the native allocation registration with ImageWriter, such that GC
can get some hint when clean up the large memory object.
* Close all pending images when closing ImageReader. This could avoid native
mem leaks for some bad app practice. For example, some apps may hold images
in background service when activity is paused/destroyed, which could cause
huge native memory leaks even ImageReader is closed.
* make Image close thread safe: it is possible the clients close the image
in listener thread and the client main thread.
* Some minor code refactor to reduce the code duplication.

Bug: 25088440
Change-Id: I37d22b52aeb8d2521bf9c702b0f54c05905473e0