ANDROID: allmodconfig: Force gki_defconfig as base

Allmodconfig enables as many options as it can to maximize the number of
modules it can build. While this is generally a good idea, this can also
lead to very convoluted configurations (such as CPU_BIG_ENDIAN=y on
arm64) which we don't necessarily want to support even if they break. On
the other hand, gki_defconfig already contains the set of core options
we definitely want to support, so it makes sense to use that as a base.

Point KCONFIG_ALLMODCONFIG to the relevant gki_defconfig file in order
to ensure a minimally sensible config that allconfig is not allowed to
modify.

Bug: 140224784
Signed-off-by: Quentin Perret <qperret@google.com>
Change-Id: Ib4cf3c9565f040a577ce3cec008293520be1af84