synchronize updateStringCache on mAccessLock

Resources.java used to synchronize on mTmpValue but has since
switched to mAccessLock. mTmpValue can be null in some instances
causing a NPE during a theme change.

Change-Id: I7411dfdf6e8e411381ae993872a3c740dad19300
Merge "synchronize updateStringCache on mAccessLock" into pac-4.4