WebmWriter:set mResumed=false in pause()

WebmFrameThread.cpp does not set the resumed state as false in pause() function.
Hence when multiple pause/start APIs are called, both the mPaused and mResumed
variables remain true. When read happens in paused state, since mResumed
is true, timestamp adjustment logic goes wrong causing an abort. So, set
mResumed=false in pause().

Bug: 146319756

Test: atest writerTest -- --enable-module-dynamic-download=true

Change-Id: I8ec246df4e1ff491c3ade6116696c53551fea11c
Merged-In: I8ec246df4e1ff491c3ade6116696c53551fea11c
(cherry picked from commit 36d77d7f)