AAPT2: Fix mkdirs implementation

Once switched to using std::string, the mkdirs implementation was
trying to create an empty string when an absolute file path on
linux or macOS was used.

Bug: 62336414
Bug: 65645766
Test: manual
Change-Id: I52f3050b410a923ca48f353b0983667c16d00ee8
Merged-In: I52f3050b410a923ca48f353b0983667c16d00ee8