Use 'index' instead of 'ttcIndex' for attribute name.

There are two reasons for this. First, the name 'ttcIndex' is over
specific, there are many indexed font containers in addition to ttc,
such as fon, pfr, bdf, and dfont. There may be others in the future.
Second, Skia implemented this attribute with the name 'index' quite
some time ago. By naming this attribute 'index' even existing apps will
be able to use this attribute (including Chrome).

BUG: 10861108
Change-Id: I68d2b69fad304d93313f96bf3365af71906dfe69