AAPT Files - Permit invariant filepaths for getting file names

AAPT compile provides a "--source-path" option that instructs aapt to
use a provided source path rather than the default source resource as
resource orgin. However, this is not file system agnostic. This causes
incorrect outputs of AAPT link, as source path parsing is tied to the
file system. This change ensures that AAPT Files::getFilename can parse
filepaths, regardless of file system filepath type.

Bug: 141301405, 155218379
Test: manually tested on windows with a unix style filepath, passes aapt
tests
Change-Id: I548de01ad513b549dc30eb35d2a59813fa3d4e69