Revert "Disable BTI for now."

This reverts commit c75994d6.

Reason for revert: BTI shipped in S, and we're going to work on app compat outreach rather than regress this security feature.

Bug: https://issuetracker.google.com/216395195
Bug: https://issuetracker.google.com/233339366
Change-Id: Ic7143d756907ceb574a686258751840ce62c920f
Merged-In: Ic7143d756907ceb574a686258751840ce62c920f