ueventd: scans /apex/*/firmware for firmwares

In addition to "firmware_directories", ueventd scans /apex/*/firmware/
directory as well to find firmware files.

Bug: 167942098
Test: loading firmware from vibrator apex successfully.
      (sunfish)
Change-Id: I90fc8f9ad843a08b1ca98a2be1b5d22c0c5954a3