As reported in http://b/issue?id=1398215 MemoryFile did not
munmap(2) the ashmem region after closing it. This
causes the process to leak virtual address space.
This change fixes the problem by calling munmap(2) in
close(). The unmapping is done by a helper method deactivate().
The change also replaces the use of an int for the
file descriptor with a FileDescriptor object to
make sure that we keep track of when the file descriptor
has been closed. I chose to implement it this way because I
will need decativate() and a FileDescriptor object in an
upcoming change that allows sending MemoryFile file
descriptors between processes.
The change also adds a number of tests for the behavior
of close(). The testCloseRead() and testCloseWrite() fail
with the old MemoryFile implementation, and testCloseLeak()
causes a segfault. They all pass now.
761e0918d3