Make AppCompat's night mode work nicely on M+

AppCompat currently clobbers the system night mode
which is not ideal. This CL fixes AppCompat to work
nicely with the system's night mode, especially when
using AUTO.

FOLLOW_SYSTEM now does the correct thing on all
platforms now but just letting the system handle
everything.

BUG: 27626356
Change-Id: I218e133c8c1e1a09c550cceecea3d1922a1ad423
3 files changed