stagefright aacenc: Safeguard against overwriting bits

Previously, if bits above the lowest noBitsToWrite were
set, they would be ORed into the previous cache word,
setting unrelated bits erroneously.

This doesn't noticeably affect the performance of the
codec as a whole.

Change-Id: Ie9935533c4299b8f07cb14485f039a9be9c84016