Report build_type to Omaha.

Send additional os_build_type <app> attribute to distinguish between
user and userdebug images.

Bug: 36491462
Test: check omaha request xml
Change-Id: I519e85293db9a2194e0cd015a3a944adc723c3dd
(cherry picked from commit eba2297072914582aed5008ad5f99374124ae4d7)