build: Don't run backuptool on GMS builds

Change-Id: I5dde27f9d16b88049171db9805221d92e67f3e5d
Store the path to the ota file generator where it can be reused

Change-Id: I8896713c79a751b79fbbcc75f6ba30dad4ea9fac
Don't make assumptions about the formats of build descriptions

Change-Id: Id83a7594e9e1b9b4ffbdbaba695506d8d0d21a46