base

Add importantForAccessibility mode to block entire hierarchy

Since older versions of the platform will crash if the mode is
set to 3, this CL adds an extra bit and uses 4 as the mode. This
will also cleanly default the mode to AUTO on unsupported
platforms.

BUG: 11031947
Change-Id: I48035abbf35523d65cead6f27be10f8087973ceb