Add system_ext partition to aconfig_flags.pb paths

aconfig flags for the system_ext container are added to
/system_ext/etc/aconfig_flags.pb. Add this path to the list of the
aconfig text profiles.

Bug: 385213580
Test: "adb shell aflags list" must not be changed.
Change-Id: I0307caf0312b3e5ac920677a32e7e9712159f7d4