expected.h - fix bugprone-branch-clone warning

Fixes:
  system/core/base/include/android-base/expected.h:606:39: warning: repeated branch in conditional chain [bugprone-branch-clone]
    if (x.has_value() != y.has_value()) {
                                        ^
  system/core/base/include/android-base/expected.h:608:4: note: end of the original
    } else if (!x.has_value()) {
     ^
  system/core/base/include/android-base/expected.h:610:10: note: clone 1 starts here
    } else {
           ^

Test: builds
Bug: 153035880
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Ie67a8bb1bf622319adea15466c42077e0e9b1a18