ginstall: Add support for Android images

Design doc: go/ginstall-on-atv

Noteable changes (for tests):
* the "hnvram" binary now handles reading and writing of attributes
* I added an actual (non-Android) gfhd254 test in install_test.sh
* some renaming of testdata files to make things clearer

Change-Id: I08cd178efab440939e3a8560d253e0109f4ddc64
22 files changed