Allow LogBuffers to be cleared

Especially for memory tests (which run ~100 iterations without restart of SysUI), it would be great to be able to flush the LogBuffers - right now the size of logbuffers increases through iterations which ends up as false positive leaks.
This CL adds an ADB command to clear the log buffers to be executed by the test runner for performance tests.

Bug: 437098294
Flag: EXEMPT logging only
Test: built SysUI, ran tests
Change-Id: I3097883b52b74dcd33dabd7ad0e81d023348e7ab