Disable kythe indexing of R files

https://r.android.com/3207856 special-cased kythe builds to always
generate R.srcjar files. Some of the R.java files are too big to be
processed by kythe (b/397132972#comment43), so disable R.srcjar generation.

Bug: 397132972
Test: XREF_CORPUS=blah m nothing #verified that R.srcjar is not present
in .ninja files

Change-Id: Ifadf6418c221adb144b10212becac1d6901e951f