init/util.cpp: don't return a negative unsigned value

android_name_to_id() returns -1U on error, which causes a
crash when the following clang options are enabled:

  -fsanitize=signed-integer-overflow,unsigned-integer-overflow
  -ftrap-function=abort
  -fsanitize-undefined-trap-on-error

Rather than returning a negative unsigned value (which doesn't
make a lot of sense, IMHO), return a positive unsigned value.

While we're here, add logging on decode_uid failures.

Bug: 21880301
Change-Id: I652e4c1daa07c7494cceca2b4e1656b9158f2604