adb: reorganize adbd USB handling code.

Break adbd's USB initialization code into its own static library to
allow it to be used by benchmark code that pretends to be adbd.

Bug: http://b/31289465
Test: adb shell works on bullhead
Change-Id: I0ac7d78a4f7aef59bd6d14fc5cf1ed870e1f3a9d
3 files changed