Add SandboxedActivity class to the sandbox app

The activity while onCreate, it will notify the SDK about the activity
by passing the activity and the token from the intent extras to the
SdkSandboxActivityRegistry.

Test: atest SdkSandboxUnitTests
Bug: 263471515
Change-Id: Idb18b59f74af9b58f4cb30b5c0f6c0d24ed143c8
4 files changed