AAPT2: Fix resource table load time regression

A previous change (deee3950) caused duplicate entries to be created for
entries eith entry ids greater than 0x0ff. This is because the wrong
data type was used (uint8_t instead of uint16_t). This made loading in
resources slower as well since more entries had to be iterated over.

Bug: 36051266
Test: Dumping all resources in 700 apks found in the android tree took 1
minute instead of 5 minutes. Created a test in aapt2_tests.

Change-Id: I1c3d830da517a56ac3496221dbe605c72e0c6014