C2SoftAvc: fix flush and map error

- Fix infinite loop at flush.
- Allocate flush buffer at setFlushMode().
- Handle input buffer mapping error.
- Remove unused code.

Bug: 74073134
Test: adb shell setprop debug.stagefright.ccodec yes
Test: adb shell setprop debug.stagefright.omx_default_rank 1000
Test: adb shell killall mediaserver
Test: atest android.media.cts.MediaCodecTest
Merged-In: I6d0187fcd1d9ff2cf90bbf79d21ead1fba5df5a6
Change-Id: I6d0187fcd1d9ff2cf90bbf79d21ead1fba5df5a6
(cherry picked from commit 21914e52)