Add an optional flag for an assets/ dir.

This allows supplemental resource files (e.g. NOTICE output) to be
included in the APEX under assets.

Bug: 135218846
Test: Built Mainline modules + manual inspection
Change-Id: Iafdbc9012ea72a2298b84e70779282c00f191f31
Merged-In: Iafdbc9012ea72a2298b84e70779282c00f191f31
(cherry picked from commit 2247d1d56781e832f79afe82637e725226a272d3)