Make backup_flags exportable and export enable_cross_platform_transfer

For the build system to recognize that a flag declaration contains
exported flags, we first have to mark the build target as exportable.

The enable_cross_platform_transfer flag is set to exported so it can be
accessed from google3.

See go/android-flag-containers#exported-mode

Bug: 438172201
Test: make sdk dist; then copied android-flags.jar to google3 and
  successfully compiled code depending on the new exported flag.
Flag: NONE changes to build targets cannot be flagged

Change-Id: I391be577330878d31993289fe56c844bc5712fcc