ANDROID: gki_defconfig: Enable CONFIG_PROVE_LOCKING for android-mainline

This patch enables CONFIG_PROVE_LOCKING in android-mainline
in order to raise awareness in early device development of
lockdep warnings.

Lockdep is a very useful debug tool to find locking problems,
before they are actually hit. However it can only find one issue
at a time (it disables itself after it trips a warning). Thus if
folks ignore lockdep warnings in one area of code, it makes the
tool unusable to find real locking problems in other areas of
code.

Additionally, having it enabled in android-mainline can ensure
we catch and report upstream lockdep regressions quickly without
doubling our config testing matrix.

So while this should be disabled for android-LTS branches, lets
turn it on for android-mainline.

Bug: 300341704
Change-Id: I6466774ca8dd7d2bbbb100e77255a83db5472fec
Signed-off-by: John Stultz <jstultz@google.com>