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Add a build id field to the header structure in blob cache. Add build
id support with reading and writing the cache. When the cache gets
written it writes the build id at the end of the header. When read it
checks to see if there is a match between the current version and the
version in the cache. If not, it invalidates the cache which would
typically only occur during an ota update.  Also remove blob cache
from the host build.

bug: 18262905

Change-Id: I753b1de1986703a4c1c8691b9d2bb533b2546143