Specify the API level via the triple instead of __ANDROID_API__.

Clang derives the value of __ANDROID_API__ from the triple these days. In a
future version of clang I plan to start making the behaviour of the HWASAN pass
dependent on the API level in the triple, so it's going to need to be accurate.

Test: walleye-userdebug boots
Change-Id: Ie5e36b5c8f6dcda084cc12b1160abbdf94765174