Fix typo in APerformanceHint_borrowSessionFromJava

The @return description for APerformanceHint_borrowSessionFromJava
incorrectly stated it returns an APerformanceHintManager. This change
corrects it to APerformanceHintSession, matching the actual return type.

Bug: none
Test: N/A
Flag: EXEMPT docs change
Change-Id: Iabe208e6b0515a90b4a955149768f16bb1ea368f