contexthub: Expose CHRE version information

Add chrePlatformId and chreVersion fields to context hub metadata
returned by getHubs(). This information will be used to make
compatibility decisions at runtime. Also, add a brief description of
CHRE to the IContexthub documentation to help clarify the
relationship between the HAL and these concepts in the CHRE API.

Bug: 31402693
Test: compile test only, this only adds fields to HIDL structures
Change-Id: Iea2436ccc23b47c4cf06040bebe1c550fe58ccb7