Allow building generic OTA packages.

We already have targets that build generic system images, which can be
applied (flashed) onto matching devices to replace their target-specific
system images. This CL adds PRODUCT_BUILD_GENERIC_OTA_PACKAGE that
allows building generic OTA packages to be installed over-the-air.

Since A/B and non-A/B OTAs have different package formats, currently the
support is limited to targets that use A/B OTAs. Note that this CL only
allows _building_ the package - will need additional changes for the
actual package install as well as targeting matching devices.

Bug: 122851610
Test: `m otapackage` on a target that sets
      `PRODUCT_BUILD_GENERIC_OTA_PACKAGE := true`.
Test: TreeHugger
Change-Id: If6fd2da15d24c5aaee09618efe94514c6d83292d