apex: prebuilt_firmware installs in /etc/firmware

Installation path of prebuilt_firmware varies accoding to target
partitions. When it is for system, it installs a file in
/system/etc/firmware while it installs in /vendor/firmware for vendor.

We'd better be consistent about installation path when it is
for APEXes regardless of target partition. Otherwise, ueventd would need
to scan both /apex/*/etc/firmware and /apex/*/firmware.

Having /etc prefix for prebuilt modules helps module owners to predict
the layout of the contents.

Bug: 162701747
Bug: 167942098
Test: soong tests
Test: loading vibrator firmware from vibrator apex (sunfish)
Change-Id: I7a7105026426f8a7a156bc947304930f761c81f3