simpleperf: adjust mapped buffer size in record command.

Instead of fixing the mapped buffer size, we can adjust
it at runtime. So users don't need to adjust -m manually.

Bug: 29574526
Change-Id: Icb580df3d60f8d2cf554c0d4139e6f7f64b19f8f
Test: run simpleperf_unit_test.
5 files changed