fastboot: Warn when flashing dynamic partitions in the bootloader.

On retrofit devices, it is easy to accidentally overwrite
system/vendor/product by flashing system in the bootloader. The reason
is that GPT system_a is really the super partition, and the bootloader
doesn't know it.

Addressing this in bootloaders would require two separate commands: one
that rejects flashing system/vendor/product, and another for
expert/factory use that would allow direct flashing.

This patch introduces protection into the host fastboot tool instead.
It's not mutually exclusive with bootloader changes; having protection
in the host tool affords us better and consistent UI. However it does
rely on users having newer builds.

With this change, the following will not work in the bootloader:

    fastboot flash system       # or vendor, product, etc

The message is the same whether or not the device is a retrofit. To
continue anyway, you can do:

    fastboot flash --force system

If we decide on bootloader protection as well, the --force flag can be
re-used.

Bug: 119689480
Test: fastboot flash system # disallowed in bootloader, allowed in fastbootd
      fastboot flash --force system # allowed in bootloader
Change-Id: I0861e3f28a15be925886d5c30c7ebd4b20c477cf