Add missing stdbool.h to sensor.h.

This got hacked into the NDK headers ages ago, but never got
upstreamed to the frameworks.

Test: make checkbuild
Bug: https://github.com/android-ndk/ndk/issues/370
Merged-In: Id38cbc96633c1c61de36996c2dd9e6c2331bd6e7
Change-Id: Id38cbc96633c1c61de36996c2dd9e6c2331bd6e7