Add simple caching to XML compilation

XML compilation stage will perform many (expensive) resource ID
lookups - for the same entries. Adding this simple cache can
dramatically improve performance of this stage (often a bottleneck)

Updated: fixed for Mac OS X (also tested on Windows, Linux)

back ported from http://review.cyanogenmod.com/#/c/24040/



Change-Id: Ib1ab82ba9bf901bc2a3f9027b47d680861d6c315
Signed-off-by: Jorrit Jongma <jorrit@jongma.org>

Conflicts:

	tools/aapt/Android.mk
	tools/aapt/ResourceTable.cpp