Implement full data backup through transport

Currently no timed/scheduled full-data backup operations are
performed by the OS, but the plumbing is now in place and can
be tested using 'adb shell bmgr fullbackup pkg [pkg2 pkg3 ...]'.

The LocalTransport test transport implementation has been augmented
to support the new full-data backup API as well.

In addition, 'adb backup' now takes the -compress/-nocompress
command line options to control whether the resulting archive is
compressed before leaving the device.  Previously the archive was
always compressed.  (The default is still to compress, as it will
usually reduce the archive size considerably.)

Internally, the core implementation of gathering the full backup
data stream from the target application has been refactored into
an "engine" component that is shared by both 'adb backup' and the
transport-oriented full backup task.  The archive file header
generation, encryption, and compression logic are now factored out
of the engine itself instead of being hardwired into the data
handling.

Bug 15329632

Change-Id: I4a044faa4070d684ef457bd3e11771198cdf557c