Enable CFI for the amrwbenc software encoder static lib.

Adds LOCAL_SANITIZE := cfi to the amrwbenc static library that is
included in libstagefright_soft_amrwbenc.so. This fixes a runtime
crash with the software encoder.

Bug: 30227045
Test: 'ENABLE_CFI=true m -j40' to build with CFI, then run a GCE AVD
instance that uses the software encoder.
Change-Id: I06721597527e31f33c12baaac4feb409a3fdd0bd