uml: init: add USER_MODE_LINUX cflag, USER_MODE_LINUX case in init.cpp

Modified Android.mk to define cflag "USER_MODE_LINUX" if
TARGET_USER_MODE_LINUX := true in BoardCofig.mk.
Modified set_mmap_rnd_bits_action to return 0 if "USER_MODE_LINUX" is
defined. This is needed since uml does not support the mmap_rnd_bits
sysctl, and init would otherwise crash without this check.

Test: manual

Bug: 32523022
Change-Id: I409ef64a1fa253bfb3f9fb59d0267be159819bb8
Signed-off-by: Quang Luong <qal@google.com>