init: Use ConsumePrefix() instead of open coding in GetBlockDeviceSymlinks()

In GetBlockDeviceSymlinks() we may need to strip the
"/devices/platform/" or "/devices/" from a string. Use the helper
ConsumePrefix() to do this, which is more convenient and readable.

This change is intended to be a no-op and just a cleanup.

Bug: 316324155
Test: Build and boot
Change-Id: I1adb1906ec37ff8f6f505abc5f26e1e3b157e608