CHRE API: Allow exception to non-reentrant rule

Per discussion, we've concluded it's best to allow a CHRE
implementation to have one exception to the non-reentrant
rule.  We update the documentation to reflect this
decision.

Bug: 31959429
Change-Id: Ibc7a1393812d975f8d9965d68f182a34f1e9024f
2 files changed