ANDROID: kbuild: align UNUSED_KSYMS_WHITELIST with upstream

The UNUSED_KSYMS_WHITELIST feature as been merged in ACK as FROMLIST v5,
but the v6 got recently queued with minor changes.

Align the Android code with what will be merged upstream.

Bug: 148277666
Fixes: 3d0431a8 ("BACKPORT: FROMLIST: kbuild: allow symbol
whitelisting with TRIM_UNUSED_KSYMS")
Signed-off-by: Quentin Perret <qperret@google.com>
Change-Id: I41726e3fba6e299bb541f93ae9ac31add0c08b60