V4L2EncodeComponent: allow user to disable VBR

This patch adds a Android system property persist.vendor.v4l2_codec2.disable_vbr.
The property allows user to disable option to encode with variable bitrate
in runtime.

This change is required for a temporary workaround for b/235771157.

Bug: 235771157, 238835187
Test: android.video.cts.VideoEncoderDecoderTest#testVp8Other0Perf0640x0360
Change-Id: I4a9931431354193968f8255d0d501aed02ee3508
(cherry picked from commit f9f4b5681ff4fc22cd83192b17693d90922134c7)