pitch_detector: Use continuous recording, even if the algorithm is too slow for that...

pitch_detector: Use continuous recording, even if the algorithm is too slow for that and record a whole buffer even if the min frequency is higher. Use the minimum samplerate that allows C-4186 to be detected (usually 11.025kHz, which can reduce computational load to 1/16 compared to 44.1kHz). Get rid of 64-bit multiplies when calculating input RMS value. Stop audio playback when entering plugin. Better backlight and CPU frequency handling. audio_sample_type->int16_t. simpler buffer size rounding.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@26005 a1c6a512-1295-4272-9138-f99709370657