Fix loaded apk string pool order

Loading in an APk changed the ordering of strings in the string pool.
When loading an apk, assign the strings to the same index as they
are in the ResStringPool.

Bug: 118831219
Test: "aapt2 dump strings left.apk" prints in the correct order,
"aapt2 convert left.apk --output-format binary -o left_binary.apk" has
entries in the correct order, and aapt2_tests

Change-Id: I00014c02195f39c1152a110e90083d9b14e9216e