ANDROID: GKI: Add support for a GKI_DEFCONFIG_FRAGMENT

To provide more fleixbility in debugging GKI builds, create a mechanism
for adding an optional file which may override some of the build vars,
such as POST_DEFCONFIG_CMDS.

Bug: 171343315
Signed-off-by: J. Avila <elavila@google.com>
Change-Id: I9f4a7bad2c19bcdb3b494c97d7c1eea5c7f311ba