Add support for key-value packages to adb backup/restore.

For adding CTS tests for packages having key-value backup agents, we add
support for key-value backups to the adb backup/restore command.
Previously, packages not supporting fullbackup would just be skipped on
this command. Now, by adding the -keyvalue flag to the adb backup command,
packages supporting key-value will also be added to the resulting
tarball. Similarly, if the tarball being supplied to adb restore
contains data from key-value packages, it will be restored.

This will later be utilized for writing CTS tests for such packages.

Test: adb backup -includekeyvalue -all && adb restore backup.ab

Change-Id: I7b4ccfb7072d01d29a888952145d7cce90a4f59e