docs: rewrite resources documentation
Huge overhaul. Includes all new dev guides about how resources are used
on Android and how to provide alternatives, including how to handle
runtime configuration changes (restarts). Plus all new reference
docs for all the resource types (drawables, strings, menus, etc.).
Change-Id: I12e819d2c5fc11e062281d8fe442c3037e92000a
f940a1f316