ANDROID: gki_defconfig: set DEFAULT_MMAP_MIN_ADDR=32768

Set CONFIG_DEFAULT_MMAP_MIN_ADDR=32768 to conform with
existing CTS test (which had an issue that prevented
reliable detection of this setting).

Bug: 197914473
Signed-off-by: Todd Kjos <tkjos@google.com>
Change-Id: I5a86a7b11d32adf8689657e4559c1d870a5562c4