Themes: Avoid compiling icon packs every boot

Change-Id: I51c7effdd4592a302f457822a05cbebee6cde06b
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 changes I7411dfdf,I51c7effd into kitkat

* changes:
  synchronize updateStringCache on mAccessLock
  Themes: Avoid compiling icon packs every boot