libstagefright: getHFRRatio should return 1 by default

This fixes camera crashes from divide-by-zero. Also, replaced the
workaround introduced in 1b7ac643 by a proper fix.

On ENABLE_AV_ENHANCEMENTS path, getHFRRatio returns 1 when 0 is
detected, thus the default value should be 1 not 0. (1 will turn
all multiplication and division into no-op.)

Change-Id: I5020f461011d5903a5d9bea910e11a4af9948e7f