ueventd: change firmware location in apexes

To be consistent with other prebuilt modules within APEXes, the location
for firmware files is switched from /apex/*/firmware to
/apex/*/etc/firmware.

Bug: 167942098
Test: loading vibrator firmware from vibrator apex(sunfish)
Change-Id: Ia05735bf5a54482fded26525fa4a8f795dcfc029