Revert "Reland "Do not create 4-byte sequences in `ConvertUtf16ToModifiedUtf8()`""

This reverts commit 15ddcab869a11a4ba4c5fea722b03928a5501420.

Reason for revert: RBE build failures due to non-BMP
characters in path.

Bug: 192935764
Bug: 238888095
Change-Id: I541eed806d8bca53f858eefab290f65955399900
14 files changed