Marking SessionParams#setDontKillApp as public API

This change marks the long-time system API SessionParams#setDontKillApp as public, giving any installer the ability to install new splits without killing the subject app's processes.

CTS already covers this API via PackageManagerShellCommandTest#testDontKill*

Test: atest PackageManagerShellCommandTest
Bug: 267458096
API-Coverage-Bug: 219044664
Change-Id: Ie263ac431570fce7c4bb6879a48b8183189f0b5b