Allow a system property "media.stagefright.cache-params" to override cache/prefetcher

default parameters. To override specify a property

adb shell setprop media.stagefright.cache-params "4096/20480/15" to
set the low water threshold to 4096 KB, the high water threshold to 20 MB
and the keepalive interval to 15 secs.

if high and/or lowwater mark are negative, the default values are used for
the respective value.

if keep-alive interval is 0, keep-alives are disabled.

Change-Id: I89a4a06836e4a2f473d7a92b567ab07818c2f87d