C2SoftFlacEnc: Avoid mapping C2LinearBlock twice

C2LinearBlock can be mapped to C2WriteView at most once at the same
time. This CL eliminates a local C2WriteView which is used to check
the status of C2LinearBlock, to avoid mapping C2LinearBlock twice.

Bug: 168052467
Test: android.media.cts.MediaCodecTest#testFlacIdentity

Change-Id: Ie798eac9979d867fa5c398cb3f3960341569c63d
(cherry picked from commit 96e1e1c22c50e49228ead2bfe1ec5186f5570c04)