[res] Split keys and values in Theme::Entry vector

Having keys in a separate vector improves memory usage (because
of a 4-byte padding in the original struct that's gone) and
performance (much better cache utilization when searching only
through the keys)

Test: build + UTs + boot
Bug: 237583012
Change-Id: I5ed3bada42fabfc30dfe5de39946ee5bbab22899