init: Enable ANDROID_BASE_UNIQUE_FD_DISABLE_IMPLICIT_CONVERSION

From the unique_fd.h header file: "unique_fd's operator int is
dangerous, but we have way too much code that depends on it, so make
this opt-in at first."

From the Google C++ style guide: "Do not define implicit conversions."
See also go/cstyle#Implicit_Conversions.

Hence this CL that disables unique_fd::operator int().

Change-Id: I28d94755d5408f63e5819da8d1cbc285057f867f
Signed-off-by: Bart Van Assche <bvanassche@google.com>