GKI cleanup - Moved functions GKI_disable() and GKI_enable() to OSI

Added new functions mutex_global_lock() and mutex_global_unlock()
within the OSI module, and replaced GKI_disable() and GKI_enable()
with those functions.

Also, minor cleanup in the gki.h header file.

Change-Id: I5f410e3174541224fcf30f37e1524ef099c25193