ANDROID: move ANDROID_BACKPORT_RESERVED to android_kabi.h

ANDROID_BACKPORT_RESERVED is defined in android_vendor.h, which is
documented to be "for use for vendor modules", and "the use of these
fields is vendor specific."  This is *not* what is wanted for
ANDROID_BACKPORT_RESERVED, as it is intended for backports of upstream
features.  Therefore, move it to android_kabi.h which is the more
appropriate location.  Also fix the comment for ANDROID_KABI_RESERVE to
clarify that it is just for LTS backports, since otherwise it is unclear
whether to use ANDROID_KABI_RESERVE or ANDROID_BACKPORT_RESERVED.

Note that this changes the kconfig symbol for enabling the backport
reserved fields from CONFIG_ANDROID_VENDOR_OEM_DATA to
CONFIG_ANDROID_KABI_RESERVE.  This does not matter because both options
are enabled in GKI.

Bug: 233387627
Bug: 330611177
Change-Id: I807bc6002ddddc482035f520d100fd39b97cfc44
Signed-off-by: Eric Biggers <ebiggers@google.com>