In a head dump review, LogMessageImpl was seen as serveral

top objects when sorted by size.

This change will set a maximum cap on log size to 20 items for
LogMessageImpl on low_ram (go) devices. This change reduces
the memory footprint of com.android.systemui post-boot.

Test: manual
Bug:188842641
- refer to comment #48
- https://b.corp.google.com/issues/188842641#comment48

Change-Id: I0ef72c629241990e5f5ec877aade5af03626d83f