Add ComponentCaller with getUid(), getPackage() APIs for Activity

In preparation for the future CL where we're going to check activity
caller access to content URIs, sending this first CL to introduce
ComponentCaller class (which will contain that check API) and introduce
the set of two methods for querying UID and package name there, as
agreed.

To give some context, currently, if an app launches another app with
startActivity(intent), the launched app can't check if the caller had
access to content URIs provided in the intent. That is because the
launched app would have to have access to the caller's identity, which
can't be achieved for usual cases unless the caller shares their
identity or the launched app is privileged enough. Hence, this change is
one of the several changes that will mitigate this issue.

In this change, we're introducing:
1. A new ComponentCaller class that represents the calling app via
   tokens. For now, it has 2 APIs:
   - getUid() - returns the caller's UID
   - getPackage() - returns the caller's package

   In V, apps can only retrieve ComponentCaller instances belonging to
   Activity, so these 2 APIs have similar restrictions as
   Activity#getLaunchedFromUid/Package(), i.e. the caller must have
   opted in to sharing their identity, or the caller is considered
   internal.

2. Activity#getInitialCaller for apps to retrieve the initial
   caller.

Bug: 293467489
Test: atest CtsAndroidAppTestCases:android.app.cts.ShareIdentityTest
Test: atest FrameworksMockingCoreTests:android.app.activity.ActivityThreadClientTest
Test: atest FrameworksCoreTests:ObjectPoolTests
Test: atest FrameworksCoreTests:TransactionParcelTests
Change-Id: I67ce2f2c04c9639f362aca8b171f5b9baaf6513a