Disable deprecated declarations for libtextclassifier

The new flatbuffer deprecated Vector::Length. Disable the warning before
upstream fixes this issue.

Test: build
Exempt-From-Owner-Approval: Alredy merged downstream
Change-Id: I9588d9f554b0baac94a90aec645288bf82fd166d
Merged-In: I9588d9f554b0baac94a90aec645288bf82fd166d