Decrease size of LogBufferElements to 32 bytes.

- Remove virtual from the destructor.
- Remove mTag and derive it when calling getTag().
- Add a boolean mDropped to tell when a message is dropped.
- When dropping a message, and it contains valid tag data, reallocate
  the message to only contain the tag data.
- Add the packed tag to the class.

This saves about ~150K of PSS on a typical log stream since it moves the
size of the LogBufferElement from 48 bytes to 32 bytes which puts it in
a smaller bin.

Bug: 63792187

Test: Builds, unit tests pass.
Change-Id: Ia5afce343ea3d344fcecd78c648338e94f5c9312