Revert "Do not build tests with ThinLTO"

On second thought, this is actually counter-productive rather than an
optimisation. LTOing test code is usually quite cheap, however since
tests can pull in additional static libraries (e.g. to set up test
environment), we will have to create non-LTO variants for these
dependent libraries, which can be much more expensive.

This reverts commit 85d72973.

Bug: 195134194
Bug: 203737712
Change-Id: I1a105136075133a5b5895d00847d8796c419964c
Test: m GLOBAL_THINLTO=true dist device-tests platform_tests