Migrate signature files to 6.0:style=java

Preparation for adding support for record and sealed classes.

This change migrates these files to format `6.0:style=java`.

FCRS_CODE: sdkExempt
Flag: EXEMPT PURE_REFACTOR
Bug: 493237547
API-Coverage-Bug: 493237547
Test: m checkapi

Change-Id: I1d3f63086d1159b1abf7e0722d021910fc11a547