Build otacerts as a module.

Bug: 30414428
Test: `m -j dist` with aosp_taimen-userdebug. Check
      /system/etc/security/otacerts.zip available under system and
      recovery images.
Change-Id: I5abeb2da441fb3e3231e094063c2383eb3807852
Merged-In: I5abeb2da441fb3e3231e094063c2383eb3807852