limit mediaserver memory

Limit mediaserver using rlimit, to prevent it from bringing down the system
via the low memory killer.
Default max is 65% of total RAM, but can be customized via system property.

Bug: 28471206
Bug: 28615448
CYNGNOS-3020
Change-Id: Ic84137435d1ef0a6883e9789a4b4f399e4283f05
(cherry picked from commit 12e975bf)