Improve dotdot workaround when src begins with '/'

There are some source files in our tree that use some text processing to
find source files, and end up with source file names that look like:

  /../../dir/file.c

This isn't fixed by our dotdot workarounds because the filename doesn't
start with '../', so strip the initial '/' from any source files, since
they are all relative to $(LOCAL_PATH) anyways.

Bug: 32514380
Test: Build internal source with this problem
Test: Compare build-aosp_angler.ninja before/after
Change-Id: If869419c3d3ba35d55f6e19d9332515695e47d24