Add new debug feature to automatically create heap dumps.

Not yet working, unless you turn off SELinux enforcing.
We need to update SElinux to allow the system process
to give apps access to /data/system/heapdump/javaheap.bin.

Currently watching can only be enabled through the shell,
such as:

adb shell am set-watch-heap com.android.systemui 1024

The last number is the process pss size in bytes, so this is
asking us to warn if it goes about 1K which will be all the
time.

Change-Id: I2089e5db2927afca0bf01a363c6247ee5dcb26e8