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