This should improve performance, as file_contexts is slower than
genfs_contexts.

Bug: 62413700
Test: Built, flashed, and booted Sailfish.  Verified that the
files have the correct context and that wifi, web, and atrace work.

Merged-In: Ia28707ec565a0792bc882fbffe9e8ab9968535f5
Change-Id: I9546f3af3c95e3443684ae4764881b69987611ef