Add --dedup flag to generate-partition-aconfig-flag-file

When Soong creates per-module aconfig files by merging aconfig files
from dependencies $(sort) is no longer sufficient to dedup identical
flags.  Pass --dedup to aconfig dump.

Fixes: 313698230
Test: m out/target/product/vsoc_x86_64/system/etc/aconfig_flags.pb
Change-Id: Icd8a3fbff8fe0be1dbb617ac1db4bd73f35b2d2f