Modify instr and app context for sdk-in-sandbox instrumented tests

Currently, test instrumentation running with the sdk-in-sandbox flag
returns the following
 * getTargetContext() -> sdk-in-sanbdox context,
 * getContext() -> test context.

When running a sdk-in-sandbox instrumented test, we only care about the
sdk-in-sandbox context. With this cl, test instrumentation running with
the sdk-in-sandbox flag returns
 * getTargetContext() -> sdk-in-sandbox context,
 * getContext() -> sdk-in-sandbox context.

The following link shows a diff of the two contexts before (left) and
after (right) this change
go/sdk-instrumentation-contexts-diff

Bug: 271270588
Test: atest CtsSdkSandboxInprocessTests
Change-Id: I4007392b6422b15755ce6e0c0262a00fd8067882